diff --git a/src/backend/utils/time/combocid.c b/src/backend/utils/time/combocid.c
index 52b612b15a..a70c7542c9 100644
--- a/src/backend/utils/time/combocid.c
+++ b/src/backend/utils/time/combocid.c
@@ -41,6 +41,7 @@
 
 #include "postgres.h"
 
+#include "miscadmin.h"
 #include "access/htup_details.h"
 #include "access/xact.h"
 #include "utils/combocid.h"
@@ -119,7 +120,14 @@ HeapTupleHeaderGetCmax(HeapTupleHeader tup)
 	CommandId	cid = HeapTupleHeaderGetRawCommandId(tup);
 
 	Assert(!(tup->t_infomask & HEAP_MOVED));
-	Assert(TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetUpdateXid(tup)));
+	/*
+	 * Because GetUpdateXid() performs memory allocations if xmax is a
+	 * multixact we can't Assert() if we're inside a critical section. This
+	 * weakens the check, but not using GetCmax() inside one would complicate
+	 * things too much.
+	 */
+	Assert(CritSectionCount > 0 ||
+		   TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetUpdateXid(tup)));
 
 	if (tup->t_infomask & HEAP_COMBOCID)
 		return GetRealCmax(cid);