diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 970abd4..3aff387 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -3057,8 +3057,13 @@ AlterTableGetLockLevel(List *cmds)
 		}
 
 		/*
-		 * Take the greatest lockmode from any subcommand
+		 * Take the greatest lockmode from any subcommand following a monotone
+		 * hierarchy.
 		 */
+		Assert(cmd_lockmode == ShareUpdateExclusiveLock ||
+			   cmd_lockmode == ShareRowExclusiveLock ||
+			   cmd_lockmode == ExclusiveLock ||
+			   cmd_lockmode == AccessExclusiveLock);
 		if (cmd_lockmode > lockmode)
 			lockmode = cmd_lockmode;
 	}