diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 9e60bb3..9e7ae73 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -9905,6 +9905,7 @@ ATPostAlterTypeCleanup(List **wqueue, AlteredTableInfo *tab, LOCKMODE lockmode)
 		Form_pg_constraint con;
 		Oid			relid;
 		Oid			confrelid;
+		char		contype;
 		bool		conislocal;
 
 		tup = SearchSysCache1(CONSTROID, ObjectIdGetDatum(oldId));
@@ -9921,6 +9922,7 @@ ATPostAlterTypeCleanup(List **wqueue, AlteredTableInfo *tab, LOCKMODE lockmode)
 				elog(ERROR, "could not identify relation associated with constraint %u", oldId);
 		}
 		confrelid = con->confrelid;
+		contype = con->contype;
 		conislocal = con->conislocal;
 		ReleaseSysCache(tup);
 
@@ -9936,6 +9938,15 @@ ATPostAlterTypeCleanup(List **wqueue, AlteredTableInfo *tab, LOCKMODE lockmode)
 		if (!conislocal)
 			continue;
 
+		/*
+		 * When rebuilding an FK constraint that references the table we're
+		 * modifying, we might not yet have any lock on the FK's table, so get
+		 * one now.  We'll need AccessExclusiveLock for the DROP CONSTRAINT
+		 * step, so there's no value in asking for anything weaker.
+		 */
+		if (relid != tab->relid && contype == CONSTRAINT_FOREIGN)
+			LockRelationOid(relid, AccessExclusiveLock);
+
 		ATPostAlterTypeParse(oldId, relid, confrelid,
 							 (char *) lfirst(def_item),
 							 wqueue, lockmode, tab->rewrite);