diff --git a/upb/decode.c b/upb/decode.c
index e55ee588bf..2456efa92d 100644
--- a/upb/decode.c
+++ b/upb/decode.c
@@ -421,7 +421,9 @@ static bool decode_checkenum_slow(upb_Decoder* d, const char* ptr,
   // For packed fields the tag could be arbitrarily far in the past, so we
   // just re-encode the tag and value here.
   uint32_t tag = ((uint32_t)field->number << 3) | kUpb_WireType_Varint;
-  upb_Decode_AddUnknownVarints(d, msg, tag, v);
+  upb_Message* unknown_msg =
+      field->mode & kUpb_LabelFlags_IsExtension ? d->unknown_msg : msg;
+  upb_Decode_AddUnknownVarints(d, unknown_msg, tag, v);
   return false;
 }
 
@@ -1008,6 +1010,7 @@ static const char* decode_known(upb_Decoder* d, const char* ptr,
     upb_Message_Extension* ext =
         _upb_Message_GetOrCreateExtension(msg, ext_layout, &d->arena);
     if (UPB_UNLIKELY(!ext)) return decode_err(d, kUpb_DecodeStatus_OutOfMemory);
+    d->unknown_msg = msg;
     msg = &ext->data;
     subs = &ext->ext->sub;
   }
@@ -1073,7 +1076,6 @@ static const char* decode_unknown(upb_Decoder* d, const char* ptr,
       d->unknown_msg = msg;
       ptr = decode_group(d, ptr, NULL, NULL, field_number);
       start = d->unknown;
-      d->unknown_msg = NULL;
       d->unknown = NULL;
     }
     if (!_upb_Message_AddUnknown(msg, start, ptr - start, &d->arena)) {
@@ -1189,7 +1191,7 @@ upb_DecodeStatus upb_Decode(const char* buf, size_t size, void* msg,
 
   state.extreg = extreg;
   state.limit_ptr = state.end;
-  state.unknown_msg = NULL;
+  state.unknown = NULL;
   state.depth = depth ? depth : 64;
   state.end_group = DECODE_NOGROUP;
   state.options = (uint16_t)options;
diff --git a/upb/internal/decode.h b/upb/internal/decode.h
index af244dbd19..99aa5ae824 100644
--- a/upb/internal/decode.h
+++ b/upb/internal/decode.h
@@ -46,8 +46,8 @@
 typedef struct upb_Decoder {
   const char* end;          /* Can read up to 16 bytes slop beyond this. */
   const char* limit_ptr;    /* = end + UPB_MIN(limit, 0) */
-  upb_Message* unknown_msg; /* If non-NULL, add unknown data at buffer flip. */
-  const char* unknown;      /* Start of unknown data. */
+  upb_Message* unknown_msg; /* Used for preserving unknown data. */
+  const char* unknown; /* Start of unknown data, preserve at buffer flip. */
   const upb_ExtensionRegistry*
       extreg;         /* For looking up extensions during the parse. */
   int limit;          /* Submessage limit relative to end. */
@@ -120,7 +120,7 @@ const char* decode_isdonefallback_inl(upb_Decoder* d, const char* ptr,
   if (overrun < d->limit) {
     /* Need to copy remaining data into patch buffer. */
     UPB_ASSERT(overrun < 16);
-    if (d->unknown_msg) {
+    if (d->unknown) {
       if (!_upb_Message_AddUnknown(d->unknown_msg, d->unknown, ptr - d->unknown,
                                    &d->arena)) {
         *status = kUpb_DecodeStatus_OutOfMemory;
diff --git a/upb/mini_table.c b/upb/mini_table.c
index 3caebdaa4e..1dfd00904e 100644
--- a/upb/mini_table.c
+++ b/upb/mini_table.c
@@ -963,7 +963,7 @@ upb_MiniTable* upb_MiniTable_BuildWithBuf(const char* data, size_t len,
   decoder.table->field_count = 0;
   decoder.table->ext = kUpb_ExtMode_NonExtendable;
   decoder.table->dense_below = 0;
-  decoder.table->table_mask = 0;
+  decoder.table->table_mask = -1;
   decoder.table->required_count = 0;
 
   upb_MtDecoder_ParseMessage(&decoder, data, len);
@@ -986,7 +986,7 @@ upb_MiniTable* upb_MiniTable_BuildMessageSet(upb_MiniTablePlatform platform,
   ret->field_count = 0;
   ret->ext = kUpb_ExtMode_IsMessageSet;
   ret->dense_below = 0;
-  ret->table_mask = 0;
+  ret->table_mask = -1;
   ret->required_count = 0;
   return ret;
 }
@@ -1027,7 +1027,7 @@ upb_MiniTable* upb_MiniTable_BuildMapEntry(upb_FieldType key_type,
   ret->field_count = 2;
   ret->ext = kUpb_ExtMode_NonExtendable | kUpb_ExtMode_IsMapEntry;
   ret->dense_below = 2;
-  ret->table_mask = 0;
+  ret->table_mask = -1;
   ret->required_count = 0;
   ret->subs = subs;
   ret->fields = fields;
diff --git a/upb/msg_test.cc b/upb/msg_test.cc
index 33b103a87f..696e1c1f4e 100644
--- a/upb/msg_test.cc
+++ b/upb/msg_test.cc
@@ -515,4 +515,14 @@ TEST(MessageTest, MapField) {
 // }
 // FUZZ_TEST(FuzzTest, DecodeEncodeArbitrarySchemaAndPayload);
 //
+// TEST(FuzzTest, DecodeEncodeArbitrarySchemaAndPayloadRegression) {
+//   DecodeEncodeArbitrarySchemaAndPayload(
+//       {{"\256\354Rt\216\3271\234", "\243\243\267\207\336gV\366w"},
+//        {"z"},
+//        "}\212\304d\371\363\341\2329\325B\264\377?\215\223\201\201\226y\201%"
+//        "\321\363\255;",
+//        {}},
+//       "\010", -724543908, -591643538);
+// }
+//
 // end:google_only