diff --git a/src/s2/s2cell.cc b/src/s2/s2cell.cc
index 44b1f589..baba5edb 100644
--- a/src/s2/s2cell.cc
+++ b/src/s2/s2cell.cc
@@ -325,7 +325,7 @@ void S2Cell::Encode(Encoder* const encoder) const {
 
 bool S2Cell::Decode(Decoder* const decoder) {
   S2CellId id;
-  if (!id.Decode(decoder)) return false;
+  if (!id.Decode(decoder) || !id.is_valid()) return false;
   this->~S2Cell();
   new (this) S2Cell(id);
   return true;
diff --git a/src/s2/s2cell_test.cc b/src/s2/s2cell_test.cc
index dd63eee0..388a06b1 100644
--- a/src/s2/s2cell_test.cc
+++ b/src/s2/s2cell_test.cc
@@ -1011,6 +1011,17 @@ TEST(S2Cell, EncodeDecode) {
   EXPECT_EQ(orig_cell.GetBoundUV(), decoded_cell.GetBoundUV());
 }
 
+TEST(S2Cell, DecodeRejectsInvalidCellId) {
+  // Face 7 is out of range and 0 has no level marker bit.
+  for (S2CellId id : {S2CellId(~uint64_t{0}), S2CellId::None()}) {
+    Encoder encoder;
+    id.Encode(&encoder);
+    S2Cell cell;
+    Decoder decoder(encoder.base(), encoder.length());
+    EXPECT_FALSE(cell.Decode(&decoder));
+  }
+}
+
 TEST(S2Cell, GetUVCoordOfEdge) {
   // Four cells on face 0 with two boundaries each on 0/0.
   S2Cell kCell0[4] = {
diff --git a/src/s2/s2cell_union.cc b/src/s2/s2cell_union.cc
index a3df7ba3..6b912d3e 100644
--- a/src/s2/s2cell_union.cc
+++ b/src/s2/s2cell_union.cc
@@ -555,7 +555,9 @@ bool S2CellUnion::Decode(Decoder* const decoder) {
 
   vector<S2CellId> temp_cell_ids(num_cells);
   for (size_t i = 0; i < num_cells; ++i) {
-    if (!temp_cell_ids[i].Decode(decoder)) return false;
+    if (!temp_cell_ids[i].Decode(decoder) || !temp_cell_ids[i].is_valid()) {
+      return false;
+    }
   }
   cell_ids_.swap(temp_cell_ids);
   return true;
diff --git a/src/s2/s2cell_union_test.cc b/src/s2/s2cell_union_test.cc
index 2270a80d..cc73d822 100644
--- a/src/s2/s2cell_union_test.cc
+++ b/src/s2/s2cell_union_test.cc
@@ -787,6 +787,19 @@ TEST(S2CellUnion, RefuseToDecode) {
   EXPECT_FALSE(decoded_cell_union.Decode(&decoder));
 }
 
+TEST(S2CellUnion, DecodeRejectsInvalidCellId) {
+  // Same layout as S2CellUnion::Encode(), with an id on face 7.
+  Encoder encoder;
+  encoder.Ensure(sizeof(uint8_t) + 3 * sizeof(uint64_t));
+  encoder.put8(1);
+  encoder.put64(2);
+  encoder.put64(S2CellId::FromFace(1).id());
+  encoder.put64(~uint64_t{0});
+  Decoder decoder(encoder.base(), encoder.length());
+  S2CellUnion decoded_cell_union;
+  EXPECT_FALSE(decoded_cell_union.Decode(&decoder));
+}
+
 TEST(S2CellUnion, Release) {
   S2CellId face1_id = S2CellId::FromFace(1);
   S2CellUnion face1_union({face1_id});