diff --git a/idom/client/protocol.py b/idom/client/protocol.py
index bc0f032f2..daf0d1316 100644
--- a/idom/client/protocol.py
+++ b/idom/client/protocol.py
@@ -1,5 +1,5 @@
 from pathlib import Path
-from typing import Union
+from typing import Union, cast
 
 from typing_extensions import Protocol
 
@@ -11,8 +11,8 @@
 class ClientImplementation(Protocol):
     """A minimal set of functions required to use :class:`idom.widget.module.Module`"""
 
-    def register_web_module(self, name: str, source: Union[str, Path]) -> None:
-        """Add a module with the given ``name`` to the client using the given ``source``"""
+    def register_web_module(self, name: str, source: Union[str, Path]) -> str:
+        """Return the URL of a module added under the given ``name`` and contents of ``source``"""
 
     def web_module_url(self, name: str) -> str:
         """Return the URL to import the module with the given name."""
@@ -21,5 +21,7 @@ def web_module_exists(self, name: str) -> bool:
         """Check if a module with the given name is installed"""
 
 
-client_implementation: Ref[ClientImplementation] = Ref(manage)
+client_implementation: Ref[ClientImplementation] = Ref(
+    cast(ClientImplementation, manage)
+)
 """The current client implementation used by :class:`idom.widgets.module.Module`"""