diff --git a/e2e/hypermedia-ui.spec.ts b/e2e/hypermedia-ui.spec.ts
index e6ff55d..d83a989 100644
--- a/e2e/hypermedia-ui.spec.ts
+++ b/e2e/hypermedia-ui.spec.ts
@@ -58,6 +58,13 @@ const customer = {
       name: 'changeAddress', title: 'Change address', method: 'PUT', href: `${api}/customers/42/address`, type: 'application/json',
       fields: [{ name: 'address', type: 'application/json', class: [`${api}/schemas/address`] }],
     },
+    {
+      name: 'moveTo', title: 'Move to', method: 'PUT', href: `${api}/customers/42/move`, type: 'application/json',
+      fields: [{
+        name: 'address', type: 'application/json', class: [`${api}/schemas/address`],
+        value: { street: '1 Default Lane', city: 'Paris', labels: [] },
+      }],
+    },
     {
       name: 'uploadAvatar', title: 'Upload avatar', method: 'POST', href: `${api}/customers/42/avatar`, type: 'multipart/form-data',
       fields: [{ name: 'file', type: 'file', accept: 'text/plain' }],
@@ -82,7 +89,10 @@ async function installApi(page: import('@playwright/test').Page) {
         case '/schemas/address': return route.fulfill({
           contentType: 'application/json',
           body: JSON.stringify({
-            type: 'object', required: ['street'], properties: { street: { type: 'string' }, city: { type: 'string' } },
+            type: 'object', required: ['street', 'labels'], properties: {
+              street: { type: 'string' }, city: { type: 'string' },
+              tags: { type: ['array', 'null'], items: { type: 'string' } }, labels: { type: 'array', items: { type: 'string' } },
+            },
           }),
         });
         case '/documents/data': return route.fulfill({ contentType: 'application/json', body: JSON.stringify({ document: 'ordinary JSON' }) });
@@ -138,7 +148,11 @@ async function installApi(page: import('@playwright/test').Page) {
     }
     if (url.pathname === '/customers/42/address') {
       expect(request.method()).toBe('PUT');
-      expect(request.postDataJSON()).toEqual([{ address: { street: '12 Analytical Engine Way', city: 'London' } }]);
+      expect(request.postDataJSON()).toEqual([{ address: { street: '12 Analytical Engine Way', city: 'London', tags: null, labels: [] } }]);
+      return json({}, 204);
+    }
+    if (url.pathname === '/customers/42/move') {
+      expect(request.postDataJSON()).toEqual([{ address: { street: '1 Default Lane', city: 'Paris', tags: null, labels: [] } }]);
       return json({}, 204);
     }
     if (url.pathname === '/customers/42/avatar') {
@@ -184,6 +198,36 @@ test('executes an action with parameters', async ({ page }) => {
   await page.locator('button', { hasText: 'Change address' }).click();
   await page.getByLabel('street').fill('12 Analytical Engine Way');
   await page.getByLabel('city').fill('London');
+
+  // a nullable array starts as null, a non-nullable one without default value is missing
+  const tags = page.locator('formly-array-type', { hasText: 'tags' });
+  const labels = page.locator('formly-array-type', { hasText: 'labels' });
+  await expect(tags.locator('legend')).toHaveText('tags');
+  await expect(tags).toContainText('null');
+  await expect(labels.locator('legend')).toHaveText('labels *');
+  await expect(labels).toContainText('not set');
+  await expect(page.getByRole('button', { name: 'Submit' })).toBeDisabled();
+
+  // removing the last item leaves an empty array
+  await labels.locator('.add-button').click();
+  await labels.locator('.delete-button').click();
+  await expect(labels).toContainText('empty');
+
+  // a nullable array can be set back to null
+  await tags.locator('.add-button', { hasText: 'add' }).click();
+  await tags.locator('.add-button', { hasText: 'backspace' }).click();
+  await expect(tags).toContainText('null');
+
+  await page.getByRole('button', { name: 'Submit' }).click();
+  await expect(page.locator('app-parameter-action .success')).toBeVisible();
+});
+
+test('takes arrays from the default values', async ({ page }) => {
+  await openEntryPoint(page);
+  await page.getByRole('link', { name: 'customer' }).click();
+
+  await page.locator('button', { hasText: 'Move to' }).click();
+  await expect(page.locator('formly-array-type', { hasText: 'labels' })).toContainText('empty');
   await page.getByRole('button', { name: 'Submit' }).click();
   await expect(page.locator('app-parameter-action .success')).toBeVisible();
 });
diff --git a/src/app/hypermedia-view/actions-view/file-upload-action/file-upload-action.component.html b/src/app/hypermedia-view/actions-view/file-upload-action/file-upload-action.component.html
index a6fd835..aff2c60 100644
--- a/src/app/hypermedia-view/actions-view/file-upload-action/file-upload-action.component.html
+++ b/src/app/hypermedia-view/actions-view/file-upload-action/file-upload-action.component.html
@@ -49,7 +49,7 @@
 
     <div class="button-row">
       <!--place tooltip in parent because it does not work when button is disabled-->
-      <span matTooltip="Please select a file and complete all required fields." [matTooltipDisabled]="this.canSubmit()">
+      <span [matTooltip]="this.submitBlockedReason()" [matTooltipDisabled]="this.canSubmit()">
         <button (click)="onSubmit()"
           [disabled]="!this.canSubmit()"
           mat-flat-button
diff --git a/src/app/hypermedia-view/actions-view/file-upload-action/file-upload-action.component.ts b/src/app/hypermedia-view/actions-view/file-upload-action/file-upload-action.component.ts
index ce866f1..b21a39a 100644
--- a/src/app/hypermedia-view/actions-view/file-upload-action/file-upload-action.component.ts
+++ b/src/app/hypermedia-view/actions-view/file-upload-action/file-upload-action.component.ts
@@ -15,7 +15,7 @@ import {selectEffectiveGeneralSettings} from "../../../store/selectors";
 import { FormGroup } from '@angular/forms';
 import { FormlyFieldConfig } from '@ngx-formly/core';
 import { FormlyJsonschema } from '@ngx-formly/core/json-schema';
-import {createActionFormlyFields} from '../../formly-extensions';
+import {actionParameterValue, createActionFormlyFields, describeInvalidControls} from '../../formly-extensions';
 
 @Component({
     selector: 'app-file-upload-action',
@@ -111,6 +111,11 @@ export class FileUploadActionComponent implements OnInit {
 
   canSubmit = () => this.hasFiles() && this.form.valid;
 
+  submitBlockedReason = () => [
+    ...(this.hasFiles() ? [] : ['Please select a file.']),
+    ...(this.form.valid ? [] : [`Please fix: ${describeInvalidControls(this.form).join(', ')}`]),
+  ].join(' ');
+
   onSubmit() {
     if (!this.canSubmit()) {
       return;
@@ -147,7 +152,7 @@ export class FileUploadActionComponent implements OnInit {
   private doSubmit() {
     this.action.files = this.files;
     if (this.action.waheActionParameterName) {
-      this.action.parameters = this.form.value;
+      this.action.parameters = actionParameterValue(this.form, this.formlyFields);
     }
     this.actionResult= ActionResults.pending;
     this.executed = true;
diff --git a/src/app/hypermedia-view/actions-view/parameter-action/formly-types/array-type.scss b/src/app/hypermedia-view/actions-view/parameter-action/formly-types/array-type.scss
index 472abf5..e259029 100644
--- a/src/app/hypermedia-view/actions-view/parameter-action/formly-types/array-type.scss
+++ b/src/app/hypermedia-view/actions-view/parameter-action/formly-types/array-type.scss
@@ -47,3 +47,9 @@
 mat-card {
   border-top: 1px solid lightgray;
 }
+
+.state {
+  font-style: italic;
+  opacity: 0.6;
+  margin: 0 8px 8px 8px;
+}
diff --git a/src/app/hypermedia-view/actions-view/parameter-action/formly-types/array-type.ts b/src/app/hypermedia-view/actions-view/parameter-action/formly-types/array-type.ts
index 9ea8172..fb9d916 100644
--- a/src/app/hypermedia-view/actions-view/parameter-action/formly-types/array-type.ts
+++ b/src/app/hypermedia-view/actions-view/parameter-action/formly-types/array-type.ts
@@ -4,16 +4,22 @@ import {FieldArrayType, FormlyFieldConfig} from '@ngx-formly/core';
 @Component({
     selector: 'formly-array-type',
     template: ` <div>
-      <mat-card style="margin: 10px 0 0 0" appearance="raised">
+      <mat-card style="margin: 0 0 20px 0" appearance="raised">
         <div class="header-container">
           <mat-card-header class="header-content">
             @if (props.label) {
               <legend class="title">
-                {{ props.label }}
+                {{ props.label }}@if (!props['nullable']) {<span> *</span>}
               </legend>
             }
+            @if (!field.fieldGroup?.length) {
+              <span class="state">{{ isSet() ? 'empty' : props['nullable'] ? 'null' : 'not set' }}</span>
+            }
             <div style="margin-bottom: 3px; margin-left: 0px;">
               <mat-icon class="add-button" (click)="add()">add</mat-icon>
+              @if (props['nullable'] && isSet()) {
+                <mat-icon class="add-button" matTooltip="Set to null" (click)="unset()">backspace</mat-icon>
+              }
             </div>
           </mat-card-header>
           <mat-card-title class="description" style="margin-left: 18px;">
@@ -52,6 +58,21 @@ import {FieldArrayType, FormlyFieldConfig} from '@ngx-formly/core';
     standalone: false
 })
 export class ArrayTypeComponent extends FieldArrayType {
+  // an unset array is sent as null, an empty one as []
+  isSet(): boolean {
+    return this.model !== null && this.model !== undefined;
+  }
+
+  unset() {
+    for (let i = this.field.fieldGroup!.length - 1; i >= 0; i--) {
+      this.remove(i);
+    }
+    const parentModel = this.field.parent!.model as Record<string, unknown>;
+    parentModel[String(this.key)] = null;
+    this.formControl.markAsDirty();
+    this.formControl.updateValueAndValidity();
+  }
+
   getLabel(field: FormlyFieldConfig | undefined): string {
     if (field?.parent?.type === 'array' && !isNaN(Number(field?.parent?.key))) {
       return `[${field?.parent?.key}, ${field.key}]`;
diff --git a/src/app/hypermedia-view/actions-view/parameter-action/formly-types/boolean-type.ts b/src/app/hypermedia-view/actions-view/parameter-action/formly-types/boolean-type.ts
index 43bd487..5a1ebb8 100644
--- a/src/app/hypermedia-view/actions-view/parameter-action/formly-types/boolean-type.ts
+++ b/src/app/hypermedia-view/actions-view/parameter-action/formly-types/boolean-type.ts
@@ -1,4 +1,4 @@
-import {Component, OnInit} from '@angular/core';
+import {Component, OnInit, ViewEncapsulation} from '@angular/core';
 import { FormlyFieldCheckbox } from '@ngx-formly/material/checkbox';
 
 @Component({
@@ -13,6 +13,20 @@ import { FormlyFieldCheckbox } from '@ngx-formly/material/checkbox';
       {{ props.label }}
     </mat-checkbox>
   `,
+    // the form field around the checkbox is outside this component
+    styles: `
+      /* beats the 16px of .mdc-text-field--no-label .mat-mdc-form-field-infix */
+      .mat-mdc-form-field .mat-mdc-text-field-wrapper .mat-mdc-form-field-infix:has(> boolean-type) {
+        padding-top: 8px;
+        padding-bottom: 8px;
+      }
+
+      /* aligns the box with the text of other fields, the box's touch target pads it */
+      boolean-type .mat-mdc-checkbox {
+        margin-left: calc((var(--mat-checkbox-state-layer-size, 40px) - 18px) / -2);
+      }
+    `,
+    encapsulation: ViewEncapsulation.None,
     standalone: false
 })
 export class BooleanTypeComponent extends FormlyFieldCheckbox implements OnInit {
diff --git a/src/app/hypermedia-view/actions-view/parameter-action/parameter-action.component.html b/src/app/hypermedia-view/actions-view/parameter-action/parameter-action.component.html
index 013ca61..b046cec 100644
--- a/src/app/hypermedia-view/actions-view/parameter-action/parameter-action.component.html
+++ b/src/app/hypermedia-view/actions-view/parameter-action/parameter-action.component.html
@@ -24,7 +24,7 @@
         <formly-form [fields]="formlyFields" [form]="form" [model]="model"></formly-form>
       </div>
       <div class="button-row">
-        <span matTooltip="Please complete all required fields." [matTooltipDisabled]="this.canSubmit()">
+        <span [matTooltip]="this.submitBlockedReason()" [matTooltipDisabled]="this.canSubmit()">
           <button mat-flat-button type="submit" class="submit-button" color="primary" [disabled]="!this.canSubmit()">
             <mat-icon>send</mat-icon>
             Submit
diff --git a/src/app/hypermedia-view/actions-view/parameter-action/parameter-action.component.ts b/src/app/hypermedia-view/actions-view/parameter-action/parameter-action.component.ts
index df2981a..c38eadc 100644
--- a/src/app/hypermedia-view/actions-view/parameter-action/parameter-action.component.ts
+++ b/src/app/hypermedia-view/actions-view/parameter-action/parameter-action.component.ts
@@ -15,7 +15,7 @@ import {AppConfig} from 'src/app.config.service';
 import {Store} from "@ngrx/store";
 import {AppSettings, GeneralSettings} from "../../../settings/app-settings";
 import {selectEffectiveGeneralSettings} from "../../../store/selectors";
-import {createActionFormlyFields} from '../../formly-extensions';
+import {actionParameterValue, createActionFormlyFields, describeInvalidControls} from '../../formly-extensions';
 
 @Component({
     selector: 'app-parameter-action',
@@ -75,6 +75,8 @@ export class ParameterActionComponent implements OnInit {
 
   canSubmit = () => this.form.valid;
 
+  submitBlockedReason = () => `Please fix: ${describeInvalidControls(this.form).join(', ')}`;
+
   public onActionSubmitted() {
     if (!this.canSubmit()) {
       console.log('not valid');
@@ -89,7 +91,7 @@ export class ParameterActionComponent implements OnInit {
   }
 
   private doActionSubmitted = () => {
-    this.action.parameters = this.form.value;
+    this.action.parameters = actionParameterValue(this.form, this.formlyFields);
     this.actionResult = ActionResults.pending;
     this.executed = true;
 
diff --git a/src/app/hypermedia-view/formly-extensions.spec.ts b/src/app/hypermedia-view/formly-extensions.spec.ts
index 609aa7e..c66d060 100644
--- a/src/app/hypermedia-view/formly-extensions.spec.ts
+++ b/src/app/hypermedia-view/formly-extensions.spec.ts
@@ -1,10 +1,10 @@
 import {TestBed} from '@angular/core/testing';
-import {FormGroup} from '@angular/forms';
+import {FormArray, FormControl, FormGroup, Validators} from '@angular/forms';
 import {FormlyFieldConfig, FormlyFormBuilder, FormlyModule} from '@ngx-formly/core';
 import {FormlyJsonschema} from '@ngx-formly/core/json-schema';
 import {FormlyMaterialModule} from '@ngx-formly/material';
 import {JSONSchema7} from 'json-schema';
-import {createActionFormlyFields} from './formly-extensions';
+import {createActionFormlyFields, describeInvalidControls} from './formly-extensions';
 import {SchemaSimplifier} from './siren-parser/schema-simplifier';
 
 describe('createActionFormlyFields', () => {
@@ -83,3 +83,17 @@ describe('createActionFormlyFields', () => {
     });
   });
 });
+
+describe('describeInvalidControls', () => {
+  it('lists the path and failed validators of each invalid control', () => {
+    const form = new FormGroup({
+      name: new FormControl('set', Validators.required),
+      state: new FormControl(null, [Validators.required, Validators.minLength(2)]),
+      paging: new FormGroup({size: new FormControl(null, Validators.required)}),
+      tags: new FormArray([new FormControl('ok'), new FormControl(null, Validators.required)]),
+      ignored: new FormControl({value: null, disabled: true}, Validators.required),
+    });
+
+    expect(describeInvalidControls(form)).toEqual(['state (required)', 'paging.size (required)', 'tags.1 (required)']);
+  });
+});
diff --git a/src/app/hypermedia-view/formly-extensions.ts b/src/app/hypermedia-view/formly-extensions.ts
index 3a260d2..0e13b20 100644
--- a/src/app/hypermedia-view/formly-extensions.ts
+++ b/src/app/hypermedia-view/formly-extensions.ts
@@ -1,31 +1,20 @@
 
-import {AbstractControl} from '@angular/forms';
+import {AbstractControl, FormArray, FormGroup} from '@angular/forms';
 import {FormlyExtension, FormlyFieldConfig} from '@ngx-formly/core';
 import {FormlyJsonschema} from '@ngx-formly/core/json-schema';
 import {JSONSchema7} from 'json-schema';
 
 // fix forms with arrays do not allow empty arrays to be submitted (which is not null)
 export const allowEmptyArrayExtension: FormlyExtension = {
-  postPopulate(field: FormlyFieldConfig) {
-    const ctrl = field.formControl;
-
-    if (field.type === 'array' && ctrl) {
-      // Because the model is initially empty ([]) and the schema says it's required,
-      // the FormControl is born with an error.
-      // in that case remove to allow input
-      if (ctrl.hasError('required')) {
-        ctrl.clearValidators();
-
-        // Update UI so the label/asterisk updates
-        if (field.props) {
-          field.props.required = false;
-        }
-
-        // Refresh validity state
-        ctrl.updateValueAndValidity({emitEvent: false});
-
-        console.log(`Successfully bypassed 'required' for ${field.key} to allow empty arrays`);
-      }
+  // before formly evaluates expressions and adds validators, so the form is valid from the start
+  prePopulate(field: FormlyFieldConfig) {
+    if (field.type !== 'array') {
+      return;
+    }
+    // formly derives props.required from the schema's required list, and its required validator rejects []
+    delete field.expressions?.['props.required'];
+    if (field.props) {
+      field.props.required = false;
     }
   }
 };
@@ -39,10 +28,22 @@ export function createActionFormlyFields(formlyJsonschema: FormlyJsonschema, jso
         }
         const types = schemaTypes(mapSource);
         const allowsNull = isUntyped(mapSource) || isNullable(mapSource) || hasNullableBranch(mapSource);
+        const isArray = mappedField.type === 'array';
         mappedField.validators ??= {};
-        mappedField.validators['required'] = (control: AbstractControl) => (allowsNull || (control.value !== null && control.value !== undefined));
+        // a FormArray's value is always an array, only the model tells an unset array from an empty one
+        mappedField.validators['required'] = (control: AbstractControl, field: FormlyFieldConfig) =>
+          allowsNull || isSet(isArray ? field.model : control.value);
+        if (isArray && mappedField.props) {
+          // the array type shows the asterisk and null state itself, see ArrayTypeComponent
+          mappedField.props['nullable'] = allowsNull;
+        }
+        if (isArray && mapSource.default === undefined) {
+          // formly defaults arrays in the required list to [], but without a default value from the server they are not set
+          delete mappedField.defaultValue;
+        }
         // shows the asterisk, formly only sets it for properties in the schema's required list
-        if (!allowsNull && mappedField.key !== undefined && mappedField.props) {
+        // not for arrays: required would reject an empty array, which is a valid value
+        if (!allowsNull && !isArray && mappedField.key !== undefined && mappedField.props) {
           mappedField.props.required = true;
         }
         if (types.includes('string') && mapSource.format === 'date') {
@@ -57,6 +58,51 @@ export function createActionFormlyFields(formlyJsonschema: FormlyJsonschema, jso
   ];
 }
 
+// the form value, except that arrays the user never set are null instead of the FormArray's empty array
+export function actionParameterValue(form: FormGroup, fields: FormlyFieldConfig[]): Record<string, unknown> {
+  const value = structuredClone(form.value);
+  replaceUnsetArrays(fields, value);
+  return value;
+}
+
+function replaceUnsetArrays(fields: FormlyFieldConfig[] | undefined, value: unknown) {
+  if (value === null || typeof value !== 'object') {
+    return;
+  }
+  const values = value as Record<string, unknown>;
+  for (const field of fields ?? []) {
+    if (field.key === undefined || field.key === null) {
+      // e.g. the root object or a oneOf wrapper, which share the value of their parent
+      replaceUnsetArrays(field.fieldGroup, value);
+      continue;
+    }
+    const key = String(field.key);
+    if (!(key in values)) {
+      continue;
+    }
+    if (field.type === 'array' && !isSet(field.model)) {
+      values[key] = null;
+    } else {
+      replaceUnsetArrays(field.fieldGroup, values[key]);
+    }
+  }
+}
+
+function isSet(value: unknown): boolean {
+  return value !== null && value !== undefined;
+}
+
+// e.g. ["DeploymentState (enum)", "Paging.PageSize (required)"], to tell the user why a form cannot be submitted
+export function describeInvalidControls(control: AbstractControl, path = ''): string[] {
+  const own = control.errors ? [`${path || 'form'} (${Object.keys(control.errors).join(', ')})`] : [];
+  const children = control instanceof FormGroup || control instanceof FormArray
+    ? Object.entries<AbstractControl>(control.controls)
+      .filter(([, child]) => child.enabled)
+      .flatMap(([key, child]) => describeInvalidControls(child, path ? `${path}.${key}` : key))
+    : [];
+  return [...own, ...children];
+}
+
 function schemaTypes(schema: JSONSchema7): string[] {
   return schema.type === undefined ? [] : schema.type instanceof Array ? schema.type : [schema.type];
 }
diff --git a/src/app/hypermedia-view/property-tree/property-tree.component.scss b/src/app/hypermedia-view/property-tree/property-tree.component.scss
index 07e3b36..fdcef4b 100644
--- a/src/app/hypermedia-view/property-tree/property-tree.component.scss
+++ b/src/app/hypermedia-view/property-tree/property-tree.component.scss
@@ -174,7 +174,7 @@ button[mat-icon-button] {
   flex: 1;
 }
 
-::ng-deep .mat-mdc-checkbox {
+:host ::ng-deep .mat-mdc-checkbox {
   transform: scale(0.8);
 }