diff --git a/changelog.d/9631.misc b/changelog.d/9631.misc
new file mode 100644
index 0000000000..35338cd332
--- /dev/null
+++ b/changelog.d/9631.misc
@@ -0,0 +1 @@
+Add additional type hints to the Homeserver object.
diff --git a/synapse/handlers/register.py b/synapse/handlers/register.py
index 1abc8875cb..d7f226d589 100644
--- a/synapse/handlers/register.py
+++ b/synapse/handlers/register.py
@@ -437,10 +437,10 @@ class RegistrationHandler(BaseHandler):
 
                 if RoomAlias.is_valid(r):
                     (
-                        room_id,
+                        room,
                         remote_room_hosts,
                     ) = await room_member_handler.lookup_room_alias(room_alias)
-                    room_id = room_id.to_string()
+                    room_id = room.to_string()
                 else:
                     raise SynapseError(
                         400, "%s was not legal room ID or room alias" % (r,)
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index 1660921306..4d20ed8357 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -155,6 +155,10 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         """
         raise NotImplementedError()
 
+    @abc.abstractmethod
+    async def forget(self, user: UserID, room_id: str) -> None:
+        raise NotImplementedError()
+
     def ratelimit_invite(self, room_id: Optional[str], invitee_user_id: str):
         """Ratelimit invites by room and by target user.
 
diff --git a/synapse/handlers/room_member_worker.py b/synapse/handlers/room_member_worker.py
index 108730a7a1..d75506c75e 100644
--- a/synapse/handlers/room_member_worker.py
+++ b/synapse/handlers/room_member_worker.py
@@ -14,7 +14,7 @@
 # limitations under the License.
 
 import logging
-from typing import List, Optional, Tuple
+from typing import TYPE_CHECKING, List, Optional, Tuple
 
 from synapse.api.errors import SynapseError
 from synapse.handlers.room_member import RoomMemberHandler
@@ -25,11 +25,14 @@ from synapse.replication.http.membership import (
 )
 from synapse.types import Requester, UserID
 
+if TYPE_CHECKING:
+    from synapse.app.homeserver import HomeServer
+
 logger = logging.getLogger(__name__)
 
 
 class RoomMemberWorkerHandler(RoomMemberHandler):
-    def __init__(self, hs):
+    def __init__(self, hs: "HomeServer"):
         super().__init__(hs)
 
         self._remote_join_client = ReplRemoteJoin.make_client(hs)
@@ -83,3 +86,6 @@ class RoomMemberWorkerHandler(RoomMemberHandler):
         await self._notify_change_client(
             user_id=target.to_string(), room_id=room_id, change="left"
         )
+
+    async def forget(self, target: UserID, room_id: str) -> None:
+        raise RuntimeError("Cannot forget rooms on workers.")
diff --git a/synapse/server.py b/synapse/server.py
index 48ac87a124..dd4ee7dd3c 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -96,7 +96,7 @@ from synapse.handlers.room import (
     RoomShutdownHandler,
 )
 from synapse.handlers.room_list import RoomListHandler
-from synapse.handlers.room_member import RoomMemberMasterHandler
+from synapse.handlers.room_member import RoomMemberHandler, RoomMemberMasterHandler
 from synapse.handlers.room_member_worker import RoomMemberWorkerHandler
 from synapse.handlers.search import SearchHandler
 from synapse.handlers.set_password import SetPasswordHandler
@@ -630,7 +630,7 @@ class HomeServer(metaclass=abc.ABCMeta):
         return ThirdPartyEventRules(self)
 
     @cache_in_self
-    def get_room_member_handler(self):
+    def get_room_member_handler(self) -> RoomMemberHandler:
         if self.config.worker_app:
             return RoomMemberWorkerHandler(self)
         return RoomMemberMasterHandler(self)