diff --git a/python/ray/worker.py b/python/ray/worker.py
index d28fb7f94..273db9be7 100644
--- a/python/ray/worker.py
+++ b/python/ray/worker.py
@@ -1391,6 +1391,9 @@ def register_custom_serializer(cls,
         use_dict: Deprecated.
         class_id (str): Unique ID of the class. Autogenerated if None.
     """
+    worker = global_worker
+    worker.check_connected()
+
     if use_pickle:
         raise DeprecationWarning(
             "`use_pickle` is no longer a valid parameter and will be removed "