Fix some test failures when frozen_dicts are enabled (#6642)

Fixes #4026

1 files changed, 2 insertions, 2 deletions

diff --git a/synapse/storage/data_stores/main/state.py b/synapse/storage/data_stores/main/state.py

index 0dc39f139c..d07440e3ed 100644
--- a/synapse/storage/data_stores/main/state.py
+++ b/synapse/storage/data_stores/main/state.py
@@ -12,7 +12,7 @@
 # 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 collections import namedtuple
 from typing import Iterable, Tuple
@@ -107,7 +107,7 @@ class StateGroupWorkerStore(EventsWorkerStore, SQLBaseStore):
         predecessor = create_event.content.get("predecessor", None)
 
         # Ensure the key is a dictionary
-        if not isinstance(predecessor, dict):
+        if not isinstance(predecessor, collections.abc.Mapping):
             return None
 
         return predecessor