diff --git a/vmoe/checkpoints/serialization.py b/vmoe/checkpoints/serialization.py
index f5b6723..96a3218 100644
--- a/vmoe/checkpoints/serialization.py
+++ b/vmoe/checkpoints/serialization.py
@@ -287,7 +287,7 @@ def _msgpack_ext_pack(x):
     return msgpack.ExtType(_MsgpackExtType.index_info, _index_info_to_bytes(x))
   if isinstance(x, (np.ndarray, jax.Array)):
     return msgpack.ExtType(_MsgpackExtType.ndarray, _ndarray_to_bytes(x))
-  if np.issctype(type(x)):
+  if isinstance(x, np.generic):
     return msgpack.ExtType(_MsgpackExtType.npscalar,
                            _ndarray_to_bytes(np.asarray(x)))
   return x