diff --git a/changelog.d/12769.misc b/changelog.d/12769.misc
new file mode 100644
index 0000000000..27bd53abe3
--- /dev/null
+++ b/changelog.d/12769.misc
@@ -0,0 +1 @@
+Tweak the mypy plugin so that `@cached` can accept `on_invalidate=None`.
diff --git a/scripts-dev/mypy_synapse_plugin.py b/scripts-dev/mypy_synapse_plugin.py
index c775865212..d08517a953 100644
--- a/scripts-dev/mypy_synapse_plugin.py
+++ b/scripts-dev/mypy_synapse_plugin.py
@@ -21,7 +21,7 @@ from typing import Callable, Optional, Type
 from mypy.nodes import ARG_NAMED_OPT
 from mypy.plugin import MethodSigContext, Plugin
 from mypy.typeops import bind_self
-from mypy.types import CallableType, NoneType
+from mypy.types import CallableType, NoneType, UnionType
 
 
 class SynapsePlugin(Plugin):
@@ -72,13 +72,20 @@ def cached_function_method_signature(ctx: MethodSigContext) -> CallableType:
 
     # Third, we add an optional "on_invalidate" argument.
     #
-    # This is a callable which accepts no input and returns nothing.
-    calltyp = CallableType(
-        arg_types=[],
-        arg_kinds=[],
-        arg_names=[],
-        ret_type=NoneType(),
-        fallback=ctx.api.named_generic_type("builtins.function", []),
+    # This is a either
+    # - a callable which accepts no input and returns nothing, or
+    # - None.
+    calltyp = UnionType(
+        [
+            NoneType(),
+            CallableType(
+                arg_types=[],
+                arg_kinds=[],
+                arg_names=[],
+                ret_type=NoneType(),
+                fallback=ctx.api.named_generic_type("builtins.function", []),
+            ),
+        ]
     )
 
     arg_types.append(calltyp)
@@ -95,7 +102,7 @@ def cached_function_method_signature(ctx: MethodSigContext) -> CallableType:
 
 
 def plugin(version: str) -> Type[SynapsePlugin]:
-    # This is the entry point of the plugin, and let's us deal with the fact
+    # This is the entry point of the plugin, and lets us deal with the fact
     # that the mypy plugin interface is *not* stable by looking at the version
     # string.
     #
diff --git a/synapse/storage/databases/main/roommember.py b/synapse/storage/databases/main/roommember.py
index 608d40dfa1..cc528fcf2d 100644
--- a/synapse/storage/databases/main/roommember.py
+++ b/synapse/storage/databases/main/roommember.py
@@ -15,6 +15,7 @@
 import logging
 from typing import (
     TYPE_CHECKING,
+    Callable,
     Collection,
     Dict,
     FrozenSet,
@@ -634,7 +635,7 @@ class RoomMemberWorkerStore(EventsWorkerStore):
         )
 
     async def get_rooms_for_user(
-        self, user_id: str, on_invalidate=None
+        self, user_id: str, on_invalidate: Optional[Callable[[], None]] = None
     ) -> FrozenSet[str]:
         """Returns a set of room_ids the user is currently joined to.