diff --git a/synapse/third_party_rules/access_rules.py b/synapse/third_party_rules/access_rules.py
index 47d9246da0..7d80b01928 100644
--- a/synapse/third_party_rules/access_rules.py
+++ b/synapse/third_party_rules/access_rules.py
@@ -473,7 +473,7 @@ async def _freeze_room_if_last_admin_is_leaving(
         )  # type: EventBase
         if not power_level_state_event:
             return
-        power_level_content = power_level_state_event.content
+        power_level_content = power_level_state_event.content.copy()
         if not isinstance(power_level_content, dict):
             # The power level content has been set to something other than a dict...
             # bail out.