diff --git a/setup.cfg b/setup.cfg
index 47acf312c..48703df87 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -35,4 +35,5 @@ exclude_lines =
     if self.debug:
     pragma: no cover
     raise AssertionError
+    raise NotImplementedError
     self.fail\(".*"\)
diff --git a/src/websockets/extensions/base.py b/src/websockets/extensions/base.py
index 060967618..6c481a46c 100644
--- a/src/websockets/extensions/base.py
+++ b/src/websockets/extensions/base.py
@@ -38,6 +38,7 @@ def decode(
             PayloadTooBig: if decoding the payload exceeds ``max_size``.
 
         """
+        raise NotImplementedError
 
     def encode(self, frame: frames.Frame) -> frames.Frame:
         """
@@ -50,6 +51,7 @@ def encode(self, frame: frames.Frame) -> frames.Frame:
             Frame: Encoded frame.
 
         """
+        raise NotImplementedError
 
 
 class ClientExtensionFactory:
@@ -69,6 +71,7 @@ def get_request_params(self) -> List[ExtensionParameter]:
             List[ExtensionParameter]: Parameters to send to the server.
 
         """
+        raise NotImplementedError
 
     def process_response_params(
         self,
@@ -91,6 +94,7 @@ def process_response_params(
             NegotiationError: if parameters aren't acceptable.
 
         """
+        raise NotImplementedError
 
 
 class ServerExtensionFactory:
@@ -126,3 +130,4 @@ def process_request_params(
                 the client aren't acceptable.
 
         """
+        raise NotImplementedError
diff --git a/src/websockets/legacy/protocol.py b/src/websockets/legacy/protocol.py
index 1b6e58efa..21784eb55 100644
--- a/src/websockets/legacy/protocol.py
+++ b/src/websockets/legacy/protocol.py
@@ -220,8 +220,6 @@ def __init__(
         # Logger or LoggerAdapter for this connection.
         if logger is None:
             logger = logging.getLogger("websockets.protocol")
-        # https://github.com/python/typeshed/issues/5561
-        logger = cast(logging.Logger, logger)
         self.logger: LoggerLike = logging.LoggerAdapter(logger, {"websocket": self})
         """Logger for this connection."""