diff --git a/changelog.d/8237.misc b/changelog.d/8237.misc
new file mode 100644
index 0000000000..29d946cde6
--- /dev/null
+++ b/changelog.d/8237.misc
@@ -0,0 +1 @@
+Fix type hints in `SyncHandler`.
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 43fc40fc2f..8728403e62 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -16,7 +16,7 @@
 
 import itertools
 import logging
-from typing import Any, Dict, FrozenSet, List, Optional, Set, Tuple
+from typing import TYPE_CHECKING, Any, Dict, FrozenSet, List, Optional, Set, Tuple
 
 import attr
 from prometheus_client import Counter
@@ -44,6 +44,9 @@ from synapse.util.caches.response_cache import ResponseCache
 from synapse.util.metrics import Measure, measure_func
 from synapse.visibility import filter_events_for_client
 
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
 logger = logging.getLogger(__name__)
 
 # Debug logger for https://github.com/matrix-org/synapse/issues/4422
@@ -244,7 +247,7 @@ class SyncResult:
 
 
 class SyncHandler(object):
-    def __init__(self, hs):
+    def __init__(self, hs: "HomeServer"):
         self.hs_config = hs.config
         self.store = hs.get_datastore()
         self.notifier = hs.get_notifier()
@@ -717,9 +720,8 @@ class SyncHandler(object):
         ]
 
         missing_hero_state = await self.store.get_events(missing_hero_event_ids)
-        missing_hero_state = missing_hero_state.values()
 
-        for s in missing_hero_state:
+        for s in missing_hero_state.values():
             cache.set(s.state_key, s.event_id)
             state[(EventTypes.Member, s.state_key)] = s
 
@@ -1771,7 +1773,7 @@ class SyncHandler(object):
         ignored_users: Set[str],
         room_builder: "RoomSyncResultBuilder",
         ephemeral: List[JsonDict],
-        tags: Optional[List[JsonDict]],
+        tags: Optional[Dict[str, Dict[str, Any]]],
         account_data: Dict[str, JsonDict],
         always_include: bool = False,
     ):
diff --git a/synapse/storage/databases/main/roommember.py b/synapse/storage/databases/main/roommember.py
index c4ce5c17c2..c46f5cd524 100644
--- a/synapse/storage/databases/main/roommember.py
+++ b/synapse/storage/databases/main/roommember.py
@@ -298,8 +298,8 @@ class RoomMemberWorkerStore(EventsWorkerStore):
         return None
 
     async def get_rooms_for_local_user_where_membership_is(
-        self, user_id: str, membership_list: List[str]
-    ) -> Optional[List[RoomsForUser]]:
+        self, user_id: str, membership_list: Collection[str]
+    ) -> List[RoomsForUser]:
         """Get all the rooms for this *local* user where the membership for this user
         matches one in the membership list.
 
@@ -314,7 +314,7 @@ class RoomMemberWorkerStore(EventsWorkerStore):
             The RoomsForUser that the user matches the membership types.
         """
         if not membership_list:
-            return None
+            return []
 
         rooms = await self.db_pool.runInteraction(
             "get_rooms_for_local_user_where_membership_is",
diff --git a/synapse/storage/databases/main/tags.py b/synapse/storage/databases/main/tags.py
index 0c34bbf21a..96ffe26cc9 100644
--- a/synapse/storage/databases/main/tags.py
+++ b/synapse/storage/databases/main/tags.py
@@ -43,7 +43,7 @@ class TagsWorkerStore(AccountDataWorkerStore):
             "room_tags", {"user_id": user_id}, ["room_id", "tag", "content"]
         )
 
-        tags_by_room = {}
+        tags_by_room = {}  # type: Dict[str, Dict[str, JsonDict]]
         for row in rows:
             room_tags = tags_by_room.setdefault(row["room_id"], {})
             room_tags[row["tag"]] = db_to_json(row["content"])
@@ -123,7 +123,7 @@ class TagsWorkerStore(AccountDataWorkerStore):
 
     async def get_updated_tags(
         self, user_id: str, stream_id: int
-    ) -> Dict[str, List[str]]:
+    ) -> Dict[str, Dict[str, JsonDict]]:
         """Get all the tags for the rooms where the tags have changed since the
         given version