diff --git a/src/backend/storage/lmgr/predicate.c b/src/backend/storage/lmgr/predicate.c
index e24a0f2fdbd..2030322f957 100644
--- a/src/backend/storage/lmgr/predicate.c
+++ b/src/backend/storage/lmgr/predicate.c
@@ -877,12 +877,17 @@ SerialAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo)
 	LWLockAcquire(SerialControlLock, LW_EXCLUSIVE);
 
 	/*
-	 * If no serializable transactions are active, there shouldn't be anything
-	 * to push out to the SLRU.  Hitting this assert would mean there's
-	 * something wrong with the earlier cleanup logic.
+	 * If 'xid' is older than the global xmin (== tailXid), there's no need to
+	 * store it, after all. This can happen if the oldest transaction holding
+	 * back the global xmin just finished, making 'xid' uninteresting, but
+	 * ClearOldPredicateLocks() has not yet run.
 	 */
 	tailXid = serialControl->tailXid;
-	Assert(TransactionIdIsValid(tailXid));
+	if (!TransactionIdIsValid(tailXid) || TransactionIdPrecedes(xid, tailXid))
+	{
+		LWLockRelease(SerialControlLock);
+		return;
+	}
 
 	/*
 	 * If the SLRU is currently unused, zero out the whole active region from