diff --git a/synapse/handlers/admin.py b/synapse/handlers/admin.py
index 2e9bf33176..d057514b67 100644
--- a/synapse/handlers/admin.py
+++ b/synapse/handlers/admin.py
@@ -491,12 +491,7 @@ class AdminHandler:
                         prev_state_events = event.get_dict().get(
                             "prev_state_events", None
                         )
-                        if prev_state_events is None:
-                            # should be impossible as prev_state_events is a required field.
-                            raise Exception(
-                                "cannot set prev_state_events for redaction event as parent has "
-                                + "no prev_state_events",
-                            )
+                        assert prev_state_events is not None, "Parent event of redaction has no `prev_state_events` which should be impossible as `prev_state_events` is a required event in MSC4242 rooms"
                     # set the prev event to the offending message to allow for redactions
                     # to be processed in the case where the user has been kicked/banned before
                     # redactions are requested.