diff --git a/upb/mini_descriptor/decode.c b/upb/mini_descriptor/decode.c
index 7b56c7d8d99aa..db15ae9a0ee87 100644
--- a/upb/mini_descriptor/decode.c
+++ b/upb/mini_descriptor/decode.c
@@ -824,8 +824,11 @@ static upb_MiniTable* upb_MtDecoder_DoBuildMiniTableWithBuf(
 
 #if UPB_FASTTABLE
   upb_DecodeFast_TableEntry fasttable[32];
-  int fasttable_size = upb_DecodeFast_BuildTable(&decoder->table, fasttable);
-  mt_size += fasttable_size * sizeof(fasttable[0]);
+  int fasttable_size = 0;
+  if (decoder->platform == kUpb_MiniTablePlatform_64Bit) {
+    fasttable_size = upb_DecodeFast_BuildTable(&decoder->table, fasttable);
+    mt_size += fasttable_size * sizeof(fasttable[0]);
+  }
 #endif
 
   upb_MiniTable* ret = upb_Arena_Malloc(decoder->arena, mt_size);
diff --git a/upb/mini_descriptor/internal/encode_test.cc b/upb/mini_descriptor/internal/encode_test.cc
index 92765d01354dd..124ce66364ce7 100644
--- a/upb/mini_descriptor/internal/encode_test.cc
+++ b/upb/mini_descriptor/internal/encode_test.cc
@@ -339,3 +339,22 @@ TEST_P(MiniTableTest, Extendible) {
   EXPECT_EQ(kUpb_ExtMode_Extendable,
             table->UPB_PRIVATE(ext) & kUpb_ExtMode_Extendable);
 }
+
+TEST(MiniTableTest, Build32BitMiniTableWithSubmessagesNoFastTableAssert) {
+  upb::Arena arena;
+  upb::MtDataEncoder e;
+
+  ASSERT_TRUE(e.StartMessage(0));
+  ASSERT_TRUE(e.PutField(kUpb_FieldType_Message, 1, 0));
+  ASSERT_TRUE(e.PutField(kUpb_FieldType_Message, 2, 0));
+  ASSERT_TRUE(e.PutField(kUpb_FieldType_Message, 3, 0));
+
+  upb::Status status;
+  upb_MiniTable* table = _upb_MiniTable_Build(e.data().data(), e.data().size(),
+                                              kUpb_MiniTablePlatform_32Bit,
+                                              arena.ptr(), status.ptr());
+
+  ASSERT_NE(nullptr, table) << status.error_message();
+  EXPECT_EQ(3, upb_MiniTable_FieldCount(table));
+  EXPECT_EQ(0xff, table->UPB_PRIVATE(table_mask));
+}