diff --git a/changelog.d/12267.misc b/changelog.d/12267.misc
new file mode 100644
index 000000000000..e43844d44ab6
--- /dev/null
+++ b/changelog.d/12267.misc
@@ -0,0 +1 @@
+Add missing type hints for storage.
diff --git a/mypy.ini b/mypy.ini
index cfe7e77c2151..d371886a8d9d 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -43,7 +43,6 @@ exclude = (?x)
    |synapse/storage/databases/main/event_federation.py
    |synapse/storage/databases/main/push_rule.py
    |synapse/storage/databases/main/roommember.py
-   |synapse/storage/databases/main/state.py
    |synapse/storage/schema/
 
    |tests/api/test_auth.py
diff --git a/synapse/storage/databases/main/state.py b/synapse/storage/databases/main/state.py
index 28460fd36427..6e99a88a803b 100644
--- a/synapse/storage/databases/main/state.py
+++ b/synapse/storage/databases/main/state.py
@@ -12,9 +12,10 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-import collections.abc
 import logging
-from typing import TYPE_CHECKING, Collection, Iterable, Optional, Set, Tuple
+from typing import TYPE_CHECKING, Dict, Iterable, Optional, Set, Tuple
+
+from frozendict import frozendict
 
 from synapse.api.constants import EventTypes, Membership
 from synapse.api.errors import NotFoundError, UnsupportedRoomVersionError
@@ -29,7 +30,7 @@
 from synapse.storage.databases.main.events_worker import EventsWorkerStore
 from synapse.storage.databases.main.roommember import RoomMemberWorkerStore
 from synapse.storage.state import StateFilter
-from synapse.types import JsonDict, StateMap
+from synapse.types import JsonDict, JsonMapping, StateMap
 from synapse.util.caches import intern_string
 from synapse.util.caches.descriptors import cached, cachedList
 
@@ -132,7 +133,7 @@ def get_room_version_id_txn(self, txn: LoggingTransaction, room_id: str) -> str:
 
         return room_version
 
-    async def get_room_predecessor(self, room_id: str) -> Optional[dict]:
+    async def get_room_predecessor(self, room_id: str) -> Optional[JsonMapping]:
         """Get the predecessor of an upgraded room if it exists.
         Otherwise return None.
 
@@ -158,9 +159,10 @@ async def get_room_predecessor(self, room_id: str) -> Optional[dict]:
         predecessor = create_event.content.get("predecessor", None)
 
         # Ensure the key is a dictionary
-        if not isinstance(predecessor, collections.abc.Mapping):
+        if not isinstance(predecessor, (dict, frozendict)):
             return None
 
+        # The keys must be strings since the data is JSON.
         return predecessor
 
     async def get_create_event_for_room(self, room_id: str) -> EventBase:
@@ -306,7 +308,9 @@ async def _get_state_group_for_event(self, event_id: str) -> Optional[int]:
         list_name="event_ids",
         num_args=1,
     )
-    async def _get_state_group_for_events(self, event_ids: Collection[str]) -> JsonDict:
+    async def _get_state_group_for_events(
+        self, event_ids: Iterable[str]
+    ) -> Dict[str, int]:
         """Returns mapping event_id -> state_group"""
         rows = await self.db_pool.simple_select_many_batch(
             table="event_to_state_groups",
@@ -521,7 +525,7 @@ def _background_remove_left_rooms_txn(
         )
 
         for user_id in potentially_left_users - joined_users:
-            await self.mark_remote_user_device_list_as_unsubscribed(user_id)
+            await self.mark_remote_user_device_list_as_unsubscribed(user_id)  # type: ignore[attr-defined]
 
         return batch_size
 
diff --git a/synapse/util/caches/__init__.py b/synapse/util/caches/__init__.py
index 1cbc180eda72..42f6abb5e1ad 100644
--- a/synapse/util/caches/__init__.py
+++ b/synapse/util/caches/__init__.py
@@ -17,7 +17,7 @@
 import typing
 from enum import Enum, auto
 from sys import intern
-from typing import Any, Callable, Dict, List, Optional, Sized
+from typing import Any, Callable, Dict, List, Optional, Sized, TypeVar
 
 import attr
 from prometheus_client.core import Gauge
@@ -195,8 +195,10 @@ def register_cache(
     )
 }
 
+T = TypeVar("T", Optional[str], str)
 
-def intern_string(string: Optional[str]) -> Optional[str]:
+
+def intern_string(string: T) -> T:
     """Takes a (potentially) unicode string and interns it if it's ascii"""
     if string is None:
         return None