diff --git a/synapse/storage/databases/main/events.py b/synapse/storage/databases/main/events.py
index 84b38f4bf2..1aa1ad7fd9 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -3513,7 +3513,9 @@ class PersistEventsStore:
             values={
                 "event_id": event_id,
                 "reason": reason,
-                "last_check": self._clock.time_msec(),
+                # `last_check` is a TEXT column, so store the timestamp as a
+                # string rather than relying on the driver to coerce an int.
+                "last_check": str(self._clock.time_msec()),
             },
         )
 
diff --git a/synapse/storage/databases/main/events_worker.py b/synapse/storage/databases/main/events_worker.py
index 65b7d251d9..c585390e7a 100644
--- a/synapse/storage/databases/main/events_worker.py
+++ b/synapse/storage/databases/main/events_worker.py
@@ -2702,7 +2702,9 @@ class EventsWorkerStore(SQLBaseStore):
                 keyvalues={"event_id": event_id},
                 values={
                     "reason": rejection_reason,
-                    "last_check": self.clock.time_msec(),
+                    # `last_check` is a TEXT column, so store the timestamp as a
+                    # string rather than relying on the driver to coerce an int.
+                    "last_check": str(self.clock.time_msec()),
                 },
             )
         self.db_pool.simple_update_txn(