diff --git a/changelog.d/12564.misc b/changelog.d/12564.misc
new file mode 100644
index 000000000..207c32246
--- /dev/null
+++ b/changelog.d/12564.misc
@@ -0,0 +1 @@
+Consistently check if an object is a `frozendict`.
diff --git a/synapse/events/utils.py b/synapse/events/utils.py
index f8d3ba545..a6c48308b 100644
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@@ -27,7 +27,6 @@ from typing import (
 )
 
 import attr
-from frozendict import frozendict
 
 from synapse.api.constants import EventContentFields, EventTypes, RelationTypes
 from synapse.api.errors import Codes, SynapseError
@@ -204,7 +203,9 @@ def _copy_field(src: JsonDict, dst: JsonDict, field: List[str]) -> None:
     key_to_move = field.pop(-1)
     sub_dict = src
     for sub_field in field:  # e.g. sub_field => "content"
-        if sub_field in sub_dict and type(sub_dict[sub_field]) in [dict, frozendict]:
+        if sub_field in sub_dict and isinstance(
+            sub_dict[sub_field], collections.abc.Mapping
+        ):
             sub_dict = sub_dict[sub_field]
         else:
             return
@@ -622,7 +623,7 @@ def validate_canonicaljson(value: Any) -> None:
         # Note that Infinity, -Infinity, and NaN are also considered floats.
         raise SynapseError(400, "Bad JSON value: float", Codes.BAD_JSON)
 
-    elif isinstance(value, (dict, frozendict)):
+    elif isinstance(value, collections.abc.Mapping):
         for v in value.values():
             validate_canonicaljson(v)
 
diff --git a/synapse/handlers/relations.py b/synapse/handlers/relations.py
index 5efb56127..b5dc9f74b 100644
--- a/synapse/handlers/relations.py
+++ b/synapse/handlers/relations.py
@@ -11,6 +11,7 @@
 # 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,
@@ -24,7 +25,6 @@ from typing import (
 )
 
 import attr
-from frozendict import frozendict
 
 from synapse.api.constants import RelationTypes
 from synapse.api.errors import SynapseError
@@ -380,7 +380,7 @@ class RelationsHandler:
             # Do not bundle aggregations for an event which represents an edit or an
             # annotation. It does not make sense for them to have related events.
             relates_to = event.content.get("m.relates_to")
-            if isinstance(relates_to, (dict, frozendict)):
+            if isinstance(relates_to, collections.abc.Mapping):
                 relation_type = relates_to.get("rel_type")
                 if relation_type in (RelationTypes.ANNOTATION, RelationTypes.REPLACE):
                     continue
diff --git a/synapse/storage/databases/main/state.py b/synapse/storage/databases/main/state.py
index 5e340bd03..18ae8aee2 100644
--- a/synapse/storage/databases/main/state.py
+++ b/synapse/storage/databases/main/state.py
@@ -12,11 +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, Dict, Iterable, Optional, Set, Tuple
 
-from frozendict import frozendict
-
 from synapse.api.constants import EventTypes, Membership
 from synapse.api.errors import NotFoundError, UnsupportedRoomVersionError
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersion
@@ -160,7 +159,7 @@ class StateGroupWorkerStore(EventsWorkerStore, SQLBaseStore):
         predecessor = create_event.content.get("predecessor", None)
 
         # Ensure the key is a dictionary
-        if not isinstance(predecessor, (dict, frozendict)):
+        if not isinstance(predecessor, collections.abc.Mapping):
             return None
 
         # The keys must be strings since the data is JSON.
diff --git a/synapse/util/frozenutils.py b/synapse/util/frozenutils.py
index 9c405eb4d..7223af1a3 100644
--- a/synapse/util/frozenutils.py
+++ b/synapse/util/frozenutils.py
@@ -11,6 +11,7 @@
 # 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
 from typing import Any
 
 from frozendict import frozendict
@@ -35,7 +36,7 @@ def freeze(o: Any) -> Any:
 
 
 def unfreeze(o: Any) -> Any:
-    if isinstance(o, (dict, frozendict)):
+    if isinstance(o, collections.abc.Mapping):
         return {k: unfreeze(v) for k, v in o.items()}
 
     if isinstance(o, (bytes, str)):