diff --git a/src/app/hypermedia-view/formly-extensions.spec.ts b/src/app/hypermedia-view/formly-extensions.spec.ts
new file mode 100644
index 0000000..609aa7e
--- /dev/null
+++ b/src/app/hypermedia-view/formly-extensions.spec.ts
@@ -0,0 +1,85 @@
+import {TestBed} from '@angular/core/testing';
+import {FormGroup} 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 {SchemaSimplifier} from './siren-parser/schema-simplifier';
+
+describe('createActionFormlyFields', () => {
+  beforeEach(() => {
+    TestBed.configureTestingModule({
+      imports: [
+        FormlyModule.forRoot({
+          // the app's custom components are not needed to build the form controls
+          types: [
+            {name: 'null', extends: 'input'},
+            {name: 'object', extends: 'formly-group'},
+            {name: 'multischema', extends: 'formly-group'},
+          ],
+        }),
+        FormlyMaterialModule,
+      ],
+    });
+  });
+
+  const enumValues = ['Running', 'Stopped'];
+  const definitions: JSONSchema7['definitions'] = {
+    State: {enum: enumValues},
+    TypedState: {type: 'string', enum: enumValues},
+  };
+
+  // runs the schema through the same simplification as schemas fetched from the server
+  function buildForm(property: JSONSchema7, model: object = {}): {form: FormGroup, field: FormlyFieldConfig} {
+    const schema: JSONSchema7 = {type: 'object', properties: {value: property}, definitions: structuredClone(definitions)};
+    new SchemaSimplifier().simplifySchema(schema);
+    const fields = createActionFormlyFields(TestBed.inject(FormlyJsonschema), schema);
+    const form = new FormGroup({});
+    TestBed.inject(FormlyFormBuilder).build({form, fieldGroup: fields, model, options: {}});
+    return {form, field: fields[0].fieldGroup![0]};
+  }
+
+  const optionalCases: [string, JSONSchema7][] = [
+    ['nullable type', {type: ['string', 'null']}],
+    ['nullable integer', {type: ['integer', 'null']}],
+    ['nullable boolean', {type: ['boolean', 'null']}],
+    ['nullable date', {type: ['string', 'null'], format: 'date'}],
+    ['nullable integer as oneOf', {oneOf: [{type: 'integer', format: 'int32'}, {type: 'null'}]}],
+    ['enum containing null without type', {enum: [...enumValues, null]}],
+    ['nullable inline enum as oneOf', {oneOf: [{type: 'null'}, {enum: enumValues}]}],
+    ['nullable enum reference as oneOf', {oneOf: [{$ref: '#/definitions/State'}, {type: 'null'}]}],
+    ['nullable typed enum reference as oneOf', {oneOf: [{$ref: '#/definitions/TypedState'}, {type: 'null'}]}],
+    ['nullable type as anyOf', {anyOf: [{type: 'string'}, {type: 'null'}]}],
+    ['schema without type', {}],
+  ];
+
+  optionalCases.forEach(([name, property]) => {
+    it(`accepts an empty value for a ${name}`, () => {
+      const {form, field} = buildForm(property);
+      expect(form.valid).toBeTrue();
+      expect(field.props?.required).toBeFalsy();
+    });
+  });
+
+  const mandatoryCases: [string, JSONSchema7, unknown][] = [
+    ['non nullable type', {type: 'string'}, 'Running'],
+    ['integer', {type: 'integer', format: 'int32'}, 0],
+    ['boolean', {type: 'boolean'}, false],
+    ['enum without null', {enum: enumValues}, 'Running'],
+    ['enum reference', {$ref: '#/definitions/State'}, 'Running'],
+    ['number or integer as anyOf', {anyOf: [{type: 'number'}, {type: 'integer'}]}, 1],
+  ];
+
+  mandatoryCases.forEach(([name, property, value]) => {
+    it(`rejects an empty value for a ${name}`, () => {
+      const {form, field} = buildForm(property);
+      expect(form.valid).toBeFalse();
+      expect(field.props?.required).toBeTrue();
+    });
+
+    it(`accepts a value for a ${name}`, () => {
+      expect(buildForm(property, {value}).form.valid).toBeTrue();
+    });
+  });
+});
diff --git a/src/app/hypermedia-view/formly-extensions.ts b/src/app/hypermedia-view/formly-extensions.ts
index 9f1fbc6..3a260d2 100644
--- a/src/app/hypermedia-view/formly-extensions.ts
+++ b/src/app/hypermedia-view/formly-extensions.ts
@@ -2,6 +2,7 @@
 import {AbstractControl} 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 = {
@@ -36,9 +37,14 @@ export function createActionFormlyFields(formlyJsonschema: FormlyJsonschema, jso
         if (mappedField.key && mappedField.props) {
           mappedField.props.label = mappedField.key + '';
         }
+        const types = schemaTypes(mapSource);
+        const allowsNull = isUntyped(mapSource) || isNullable(mapSource) || hasNullableBranch(mapSource);
         mappedField.validators ??= {};
-        mappedField.validators['required'] = (control: AbstractControl) => (types.includes('null') || (control.value !== null && control.value !== undefined));
-        const types = mapSource.type === undefined ? [] : mapSource.type instanceof Array ? mapSource.type : [mapSource.type];
+        mappedField.validators['required'] = (control: AbstractControl) => (allowsNull || (control.value !== null && control.value !== undefined));
+        // shows the asterisk, formly only sets it for properties in the schema's required list
+        if (!allowsNull && mappedField.key !== undefined && mappedField.props) {
+          mappedField.props.required = true;
+        }
         if (types.includes('string') && mapSource.format === 'date') {
           mappedField.type = 'date';
           mappedField.parsers = [
@@ -51,6 +57,24 @@ export function createActionFormlyFields(formlyJsonschema: FormlyJsonschema, jso
   ];
 }
 
+function schemaTypes(schema: JSONSchema7): string[] {
+  return schema.type === undefined ? [] : schema.type instanceof Array ? schema.type : [schema.type];
+}
+
+function isNullable(schema: JSONSchema7): boolean {
+  return schemaTypes(schema).includes('null') || !!schema.enum?.includes(null);
+}
+
+// the simplifier types anyOf wrappers as object, so their own type does not tell
+function hasNullableBranch(schema: JSONSchema7): boolean {
+  return [...(schema.oneOf ?? []), ...(schema.anyOf ?? [])].some(branch => typeof branch === 'object' && isNullable(branch));
+}
+
+// no type info, e.g. {}
+function isUntyped(schema: JSONSchema7): boolean {
+  return schema.type === undefined && schema.enum === undefined && schema.const === undefined;
+}
+
 function formatDate(date: Date) {
   const year = date.getFullYear();
   const month = date.getMonth() + 1;