diff --git a/php/ext/google/protobuf/php-upb.c b/php/ext/google/protobuf/php-upb.c
index 3879fa0201..508b7f46dc 100644
--- a/php/ext/google/protobuf/php-upb.c
+++ b/php/ext/google/protobuf/php-upb.c
@@ -11996,6 +11996,7 @@ const upb_Extension* UPB_PRIVATE(_upb_Message_Getexts)(
 
 upb_Extension* UPB_PRIVATE(_upb_Message_GetOrCreateExtension)(
     struct upb_Message* msg, const upb_MiniTableExtension* e, upb_Arena* a) {
+  UPB_ASSERT(!upb_Message_IsFrozen(msg));
   upb_Extension* ext = (upb_Extension*)UPB_PRIVATE(_upb_Message_Getext)(msg, e);
   if (ext) return ext;
   if (!UPB_PRIVATE(_upb_Message_Realloc)(msg, sizeof(upb_Extension), a))
diff --git a/ruby/ext/google/protobuf_c/ruby-upb.c b/ruby/ext/google/protobuf_c/ruby-upb.c
index a426daf84f..9578e8e976 100644
--- a/ruby/ext/google/protobuf_c/ruby-upb.c
+++ b/ruby/ext/google/protobuf_c/ruby-upb.c
@@ -11489,6 +11489,7 @@ const upb_Extension* UPB_PRIVATE(_upb_Message_Getexts)(
 
 upb_Extension* UPB_PRIVATE(_upb_Message_GetOrCreateExtension)(
     struct upb_Message* msg, const upb_MiniTableExtension* e, upb_Arena* a) {
+  UPB_ASSERT(!upb_Message_IsFrozen(msg));
   upb_Extension* ext = (upb_Extension*)UPB_PRIVATE(_upb_Message_Getext)(msg, e);
   if (ext) return ext;
   if (!UPB_PRIVATE(_upb_Message_Realloc)(msg, sizeof(upb_Extension), a))