diff --git a/starlette/endpoints.py b/starlette/endpoints.py
index 2ba68c04b..1071fc354 100644
--- a/starlette/endpoints.py
+++ b/starlette/endpoints.py
@@ -43,7 +43,7 @@ async def method_not_allowed(self, request: Request) -> Response:
 
 class WebSocketEndpoint:
 
-    encoding = None  # May be "text", "bytes", or "json".
+    encoding: typing.Optional[str] = None  # May be "text", "bytes", or "json".
 
     def __init__(self, scope: Scope, receive: Receive, send: Send) -> None:
         assert scope["type"] == "websocket"