diff --git a/src/google/protobuf-c/protobuf-c.c b/src/google/protobuf-c/protobuf-c.c
index 4771a13..15ca3f4 100644
--- a/src/google/protobuf-c/protobuf-c.c
+++ b/src/google/protobuf-c/protobuf-c.c
@@ -1483,7 +1483,8 @@ protobuf_c_message_unpack         (const ProtobufCMessageDescriptor *desc,
           }
         case PROTOBUF_C_WIRE_TYPE_START_GROUP:
         case PROTOBUF_C_WIRE_TYPE_END_GROUP:
-          PROTOBUF_C_ASSERT_NOT_REACHED ();
+          UNPACK_ERROR (("unsupported group tag at offset %u", (unsigned)(at-data))); 
+          goto error_cleanup;
         case PROTOBUF_C_WIRE_TYPE_32BIT:
           if (rem < 4)
             {