diff --git a/rust/upb/sys/mini_table/extension_registry.rs b/rust/upb/sys/mini_table/extension_registry.rs
index 1ec16dd7cc..7cb6d847f1 100644
--- a/rust/upb/sys/mini_table/extension_registry.rs
+++ b/rust/upb/sys/mini_table/extension_registry.rs
@@ -27,7 +27,6 @@ pub enum ExtensionRegistryStatus {
     Ok = 0,
     DuplicateEntry = 1,
     OutOfMemory = 2,
-    InvalidExtension = 3,
 }
 // LINT.ThenChange(//depot/google3/third_party/upb/upb/mini_table/extension_registry.h)
 
diff --git a/upb/mini_descriptor/decode.c b/upb/mini_descriptor/decode.c
index 8661a5b94f..53928e4917 100644
--- a/upb/mini_descriptor/decode.c
+++ b/upb/mini_descriptor/decode.c
@@ -888,9 +888,15 @@ static const char* upb_MtDecoder_DoBuildMiniTableExtension(
   if (!ret || count != 1) return NULL;
 
   upb_MiniTableField* f = &ext->UPB_PRIVATE(field);
+  uint32_t fieldnum = upb_MiniTableField_Number(f);
 
-  if (upb_MiniTable_FindFieldByNumber(extendee, upb_MiniTableField_Number(f)) !=
-      NULL) {
+  const uint32_t kMaxFieldNumber = (1 << 29) - 1;
+  if (fieldnum == 0 ||
+      (fieldnum > kMaxFieldNumber && !upb_MiniTable_IsMessageSet(extendee))) {
+    upb_MdDecoder_ErrorJmp(&decoder->base, "Invalid extension field number");
+  }
+
+  if (upb_MiniTable_FindFieldByNumber(extendee, fieldnum) != NULL) {
     upb_MdDecoder_ErrorJmp(&decoder->base,
                            "Extension overlaps with a known field");
   }
diff --git a/upb/mini_table/extension_registry.c b/upb/mini_table/extension_registry.c
index 92fc440c82..a2f23ea167 100644
--- a/upb/mini_table/extension_registry.c
+++ b/upb/mini_table/extension_registry.c
@@ -14,7 +14,6 @@
 #include "upb/hash/ext_table.h"
 #include "upb/mem/arena.h"
 #include "upb/mini_table/extension.h"
-#include "upb/mini_table/internal/message.h"
 #include "upb/mini_table/message.h"
 
 // Must be last.
@@ -42,14 +41,6 @@ UPB_API upb_ExtensionRegistryStatus upb_ExtensionRegistry_Add(
   uint32_t fieldnum = upb_MiniTableExtension_Number(e);
   const upb_MiniTable* extendee = upb_MiniTableExtension_Extendee(e);
 
-  const uint32_t kMaxFieldNumber = (1 << 29) - 1;
-  if (fieldnum == 0 ||
-      (fieldnum > kMaxFieldNumber && !upb_MiniTable_IsMessageSet(extendee))) {
-    return kUpb_ExtensionRegistryStatus_InvalidExtension;
-  }
-
-  UPB_ASSERT(upb_MiniTable_FindFieldByNumber(extendee, fieldnum) == NULL);
-
   if (upb_exttable_lookup(&r->exts, extendee, fieldnum) != NULL) {
     return kUpb_ExtensionRegistryStatus_DuplicateEntry;
   }
diff --git a/upb/mini_table/extension_registry.h b/upb/mini_table/extension_registry.h
index 58e3f26c50..a6c232a862 100644
--- a/upb/mini_table/extension_registry.h
+++ b/upb/mini_table/extension_registry.h
@@ -63,7 +63,6 @@ typedef enum {
   kUpb_ExtensionRegistryStatus_Ok = 0,
   kUpb_ExtensionRegistryStatus_DuplicateEntry = 1,
   kUpb_ExtensionRegistryStatus_OutOfMemory = 2,
-  kUpb_ExtensionRegistryStatus_InvalidExtension = 3,
 } upb_ExtensionRegistryStatus;
 // LINT.ThenChange(//depot/google3/third_party/upb/rust/sys/mini_table/extension_registry.rs)
 
diff --git a/upb/test/fuzz_util.cc b/upb/test/fuzz_util.cc
index 8c75fcebf4..58056d1e1a 100644
--- a/upb/test/fuzz_util.cc
+++ b/upb/test/fuzz_util.cc
@@ -137,8 +137,9 @@ void Builder::BuildExtensions(upb_ExtensionRegistry** exts) {
       if (upb_ExtensionRegistry_Lookup(*exts, ext->UPB_PRIVATE(extendee),
                                        upb_MiniTableExtension_Number(ext)))
         continue;
-      upb_ExtensionRegistry_AddArray(
+      auto status = upb_ExtensionRegistry_AddArray(
           *exts, const_cast<const upb_MiniTableExtension**>(&ext), 1);
+      UPB_ASSERT(status == kUpb_ExtensionRegistryStatus_Ok);
     }
   }
 }