diff --git a/php/ext/google/protobuf/php-upb.c b/php/ext/google/protobuf/php-upb.c
index 4fdaf1683f..db5b30d1ee 100644
--- a/php/ext/google/protobuf/php-upb.c
+++ b/php/ext/google/protobuf/php-upb.c
@@ -7098,7 +7098,7 @@ static void upb_Decoder_AddKnownMessageSetItem(
   }
   upb_Message* submsg = _upb_Decoder_NewSubMessage2(
       d, ext->ext->UPB_PRIVATE(sub).UPB_PRIVATE(submsg),
-      &ext->ext->UPB_PRIVATE(field), (upb_TaggedMessagePtr*)&ext->data);
+      &ext->ext->UPB_PRIVATE(field), &ext->data.tagged_msg_val);
   upb_DecodeStatus status = upb_Decode(
       data, size, submsg, upb_MiniTableExtension_GetSubMessage(item_mt),
       d->extreg, d->options, &d->arena);
@@ -7452,7 +7452,7 @@ const char* _upb_Decoder_DecodeKnownField(upb_Decoder* d, const char* ptr,
       _upb_Decoder_ErrorJmp(d, kUpb_DecodeStatus_OutOfMemory);
     }
     d->original_msg = msg;
-    msg = (upb_Message*)&ext->data;
+    msg = &ext->data.UPB_PRIVATE(ext_msg_val);
     if (upb_MiniTableField_IsSubMessage(&ext->ext->UPB_PRIVATE(field))) {
       ext_sub.UPB_PRIVATE(submsg) =
           &ext->ext->UPB_PRIVATE(sub).UPB_PRIVATE(submsg);
@@ -8260,7 +8260,7 @@ static void encode_ext(upb_encstate* e, const upb_Extension* ext,
       sub.UPB_PRIVATE(subenum) =
           ext->ext->UPB_PRIVATE(sub).UPB_PRIVATE(subenum);
     }
-    encode_field(e, (upb_Message*)&ext->data, &sub,
+    encode_field(e, &ext->data.UPB_PRIVATE(ext_msg_val), &sub,
                  &ext->ext->UPB_PRIVATE(field));
   }
 }
diff --git a/php/ext/google/protobuf/php-upb.h b/php/ext/google/protobuf/php-upb.h
index 3d7ed26f42..ab63bb5674 100644
--- a/php/ext/google/protobuf/php-upb.h
+++ b/php/ext/google/protobuf/php-upb.h
@@ -1244,6 +1244,56 @@ UPB_INLINE bool upb_StringView_IsEqual(upb_StringView a, upb_StringView b) {
 #include <string.h>
 
 
+#ifndef UPB_MESSAGE_INTERNAL_TYPES_H_
+#define UPB_MESSAGE_INTERNAL_TYPES_H_
+
+#include <stdint.h>
+
+// Must be last.
+
+#define UPB_OPAQUE(x) x##_opaque
+
+struct upb_Message {
+  union {
+    uintptr_t UPB_OPAQUE(internal);  // tagged pointer, low bit == frozen
+    double d;  // Forces same size for 32-bit/64-bit builds
+  };
+};
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+UPB_INLINE void UPB_PRIVATE(_upb_Message_ShallowFreeze)(
+    struct upb_Message* msg) {
+  msg->UPB_OPAQUE(internal) |= 1ULL;
+}
+
+UPB_API_INLINE bool upb_Message_IsFrozen(const struct upb_Message* msg) {
+  return (msg->UPB_OPAQUE(internal) & 1ULL) != 0;
+}
+
+UPB_INLINE struct upb_Message_Internal* UPB_PRIVATE(_upb_Message_GetInternal)(
+    const struct upb_Message* msg) {
+  const uintptr_t tmp = msg->UPB_OPAQUE(internal) & ~1ULL;
+  return (struct upb_Message_Internal*)tmp;
+}
+
+UPB_INLINE void UPB_PRIVATE(_upb_Message_SetInternal)(
+    struct upb_Message* msg, struct upb_Message_Internal* internal) {
+  UPB_ASSERT(!upb_Message_IsFrozen(msg));
+  msg->UPB_OPAQUE(internal) = (uintptr_t)internal;
+}
+
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
+#undef UPB_OPAQUE
+
+
+#endif /* UPB_MESSAGE_INTERNAL_TYPES_H_ */
+
 // Must be last.
 
 #ifdef __cplusplus
@@ -1268,6 +1318,14 @@ typedef union {
   // documentation in kUpb_DecodeOption_ExperimentalAllowUnlinked for more
   // information.
   uintptr_t tagged_msg_val;  // upb_TaggedMessagePtr
+
+  // For an extension field, we are essentially treating ext->data (a
+  // upb_MessageValue) as if it were a message with one field that lives at
+  // offset 0. This works because upb_MessageValue is precisely one value that
+  // can hold any type of data. Recall that an extension can be of any type
+  // (scalar, repeated, or message). For a message extension, that will be a
+  // single upb_Message* at offset 0 of the upb_MessageValue.
+  struct upb_Message UPB_PRIVATE(ext_msg_val);
 } upb_MessageValue;
 
 UPB_API_INLINE upb_MessageValue upb_MessageValue_Zero(void) {
@@ -2343,56 +2401,6 @@ bool UPB_PRIVATE(_upb_Message_Realloc)(struct upb_Message* msg, size_t need,
 
 #endif /* UPB_MESSAGE_INTERNAL_MESSAGE_H_ */
 
-#ifndef UPB_MESSAGE_INTERNAL_TYPES_H_
-#define UPB_MESSAGE_INTERNAL_TYPES_H_
-
-#include <stdint.h>
-
-// Must be last.
-
-#define UPB_OPAQUE(x) x##_opaque
-
-struct upb_Message {
-  union {
-    uintptr_t UPB_OPAQUE(internal);  // tagged pointer, low bit == frozen
-    double d;  // Forces same size for 32-bit/64-bit builds
-  };
-};
-
-#ifdef __cplusplus
-extern "C" {
-#endif
-
-UPB_INLINE void UPB_PRIVATE(_upb_Message_ShallowFreeze)(
-    struct upb_Message* msg) {
-  msg->UPB_OPAQUE(internal) |= 1ULL;
-}
-
-UPB_API_INLINE bool upb_Message_IsFrozen(const struct upb_Message* msg) {
-  return (msg->UPB_OPAQUE(internal) & 1ULL) != 0;
-}
-
-UPB_INLINE struct upb_Message_Internal* UPB_PRIVATE(_upb_Message_GetInternal)(
-    const struct upb_Message* msg) {
-  const uintptr_t tmp = msg->UPB_OPAQUE(internal) & ~1ULL;
-  return (struct upb_Message_Internal*)tmp;
-}
-
-UPB_INLINE void UPB_PRIVATE(_upb_Message_SetInternal)(
-    struct upb_Message* msg, struct upb_Message_Internal* internal) {
-  UPB_ASSERT(!upb_Message_IsFrozen(msg));
-  msg->UPB_OPAQUE(internal) = (uintptr_t)internal;
-}
-
-#ifdef __cplusplus
-} /* extern "C" */
-#endif
-
-#undef UPB_OPAQUE
-
-
-#endif /* UPB_MESSAGE_INTERNAL_TYPES_H_ */
-
 // Must be last.
 
 typedef struct upb_Message upb_Message;
diff --git a/ruby/ext/google/protobuf_c/ruby-upb.c b/ruby/ext/google/protobuf_c/ruby-upb.c
index 07c682741b..4acea479f1 100644
--- a/ruby/ext/google/protobuf_c/ruby-upb.c
+++ b/ruby/ext/google/protobuf_c/ruby-upb.c
@@ -7098,7 +7098,7 @@ static void upb_Decoder_AddKnownMessageSetItem(
   }
   upb_Message* submsg = _upb_Decoder_NewSubMessage2(
       d, ext->ext->UPB_PRIVATE(sub).UPB_PRIVATE(submsg),
-      &ext->ext->UPB_PRIVATE(field), (upb_TaggedMessagePtr*)&ext->data);
+      &ext->ext->UPB_PRIVATE(field), &ext->data.tagged_msg_val);
   upb_DecodeStatus status = upb_Decode(
       data, size, submsg, upb_MiniTableExtension_GetSubMessage(item_mt),
       d->extreg, d->options, &d->arena);
@@ -7452,7 +7452,7 @@ const char* _upb_Decoder_DecodeKnownField(upb_Decoder* d, const char* ptr,
       _upb_Decoder_ErrorJmp(d, kUpb_DecodeStatus_OutOfMemory);
     }
     d->original_msg = msg;
-    msg = (upb_Message*)&ext->data;
+    msg = &ext->data.UPB_PRIVATE(ext_msg_val);
     if (upb_MiniTableField_IsSubMessage(&ext->ext->UPB_PRIVATE(field))) {
       ext_sub.UPB_PRIVATE(submsg) =
           &ext->ext->UPB_PRIVATE(sub).UPB_PRIVATE(submsg);
@@ -8260,7 +8260,7 @@ static void encode_ext(upb_encstate* e, const upb_Extension* ext,
       sub.UPB_PRIVATE(subenum) =
           ext->ext->UPB_PRIVATE(sub).UPB_PRIVATE(subenum);
     }
-    encode_field(e, (upb_Message*)&ext->data, &sub,
+    encode_field(e, &ext->data.UPB_PRIVATE(ext_msg_val), &sub,
                  &ext->ext->UPB_PRIVATE(field));
   }
 }
diff --git a/ruby/ext/google/protobuf_c/ruby-upb.h b/ruby/ext/google/protobuf_c/ruby-upb.h
index 529e498cca..6e31636b0b 100755
--- a/ruby/ext/google/protobuf_c/ruby-upb.h
+++ b/ruby/ext/google/protobuf_c/ruby-upb.h
@@ -1246,6 +1246,56 @@ UPB_INLINE bool upb_StringView_IsEqual(upb_StringView a, upb_StringView b) {
 #include <string.h>
 
 
+#ifndef UPB_MESSAGE_INTERNAL_TYPES_H_
+#define UPB_MESSAGE_INTERNAL_TYPES_H_
+
+#include <stdint.h>
+
+// Must be last.
+
+#define UPB_OPAQUE(x) x##_opaque
+
+struct upb_Message {
+  union {
+    uintptr_t UPB_OPAQUE(internal);  // tagged pointer, low bit == frozen
+    double d;  // Forces same size for 32-bit/64-bit builds
+  };
+};
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+UPB_INLINE void UPB_PRIVATE(_upb_Message_ShallowFreeze)(
+    struct upb_Message* msg) {
+  msg->UPB_OPAQUE(internal) |= 1ULL;
+}
+
+UPB_API_INLINE bool upb_Message_IsFrozen(const struct upb_Message* msg) {
+  return (msg->UPB_OPAQUE(internal) & 1ULL) != 0;
+}
+
+UPB_INLINE struct upb_Message_Internal* UPB_PRIVATE(_upb_Message_GetInternal)(
+    const struct upb_Message* msg) {
+  const uintptr_t tmp = msg->UPB_OPAQUE(internal) & ~1ULL;
+  return (struct upb_Message_Internal*)tmp;
+}
+
+UPB_INLINE void UPB_PRIVATE(_upb_Message_SetInternal)(
+    struct upb_Message* msg, struct upb_Message_Internal* internal) {
+  UPB_ASSERT(!upb_Message_IsFrozen(msg));
+  msg->UPB_OPAQUE(internal) = (uintptr_t)internal;
+}
+
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
+#undef UPB_OPAQUE
+
+
+#endif /* UPB_MESSAGE_INTERNAL_TYPES_H_ */
+
 // Must be last.
 
 #ifdef __cplusplus
@@ -1270,6 +1320,14 @@ typedef union {
   // documentation in kUpb_DecodeOption_ExperimentalAllowUnlinked for more
   // information.
   uintptr_t tagged_msg_val;  // upb_TaggedMessagePtr
+
+  // For an extension field, we are essentially treating ext->data (a
+  // upb_MessageValue) as if it were a message with one field that lives at
+  // offset 0. This works because upb_MessageValue is precisely one value that
+  // can hold any type of data. Recall that an extension can be of any type
+  // (scalar, repeated, or message). For a message extension, that will be a
+  // single upb_Message* at offset 0 of the upb_MessageValue.
+  struct upb_Message UPB_PRIVATE(ext_msg_val);
 } upb_MessageValue;
 
 UPB_API_INLINE upb_MessageValue upb_MessageValue_Zero(void) {
@@ -2345,56 +2403,6 @@ bool UPB_PRIVATE(_upb_Message_Realloc)(struct upb_Message* msg, size_t need,
 
 #endif /* UPB_MESSAGE_INTERNAL_MESSAGE_H_ */
 
-#ifndef UPB_MESSAGE_INTERNAL_TYPES_H_
-#define UPB_MESSAGE_INTERNAL_TYPES_H_
-
-#include <stdint.h>
-
-// Must be last.
-
-#define UPB_OPAQUE(x) x##_opaque
-
-struct upb_Message {
-  union {
-    uintptr_t UPB_OPAQUE(internal);  // tagged pointer, low bit == frozen
-    double d;  // Forces same size for 32-bit/64-bit builds
-  };
-};
-
-#ifdef __cplusplus
-extern "C" {
-#endif
-
-UPB_INLINE void UPB_PRIVATE(_upb_Message_ShallowFreeze)(
-    struct upb_Message* msg) {
-  msg->UPB_OPAQUE(internal) |= 1ULL;
-}
-
-UPB_API_INLINE bool upb_Message_IsFrozen(const struct upb_Message* msg) {
-  return (msg->UPB_OPAQUE(internal) & 1ULL) != 0;
-}
-
-UPB_INLINE struct upb_Message_Internal* UPB_PRIVATE(_upb_Message_GetInternal)(
-    const struct upb_Message* msg) {
-  const uintptr_t tmp = msg->UPB_OPAQUE(internal) & ~1ULL;
-  return (struct upb_Message_Internal*)tmp;
-}
-
-UPB_INLINE void UPB_PRIVATE(_upb_Message_SetInternal)(
-    struct upb_Message* msg, struct upb_Message_Internal* internal) {
-  UPB_ASSERT(!upb_Message_IsFrozen(msg));
-  msg->UPB_OPAQUE(internal) = (uintptr_t)internal;
-}
-
-#ifdef __cplusplus
-} /* extern "C" */
-#endif
-
-#undef UPB_OPAQUE
-
-
-#endif /* UPB_MESSAGE_INTERNAL_TYPES_H_ */
-
 // Must be last.
 
 typedef struct upb_Message upb_Message;