diff --git a/src/backend/optimizer/plan/subselect.c b/src/backend/optimizer/plan/subselect.c
index b80427f041..c999fb6419 100644
--- a/src/backend/optimizer/plan/subselect.c
+++ b/src/backend/optimizer/plan/subselect.c
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/subselect.c,v 1.142 2008/10/21 20:42:53 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/subselect.c,v 1.143 2008/12/08 00:16:09 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1097,6 +1097,12 @@ convert_EXISTS_sublink_to_join(PlannerInfo *root, SubLink *sublink,
 	if (!simplify_EXISTS_query(subselect))
 		return false;
 
+	/*
+	 * The subquery must have a nonempty jointree, else we won't have a join.
+	 */
+	if (subselect->jointree->fromlist == NIL)
+		return false;
+
 	/*
 	 * Separate out the WHERE clause.  (We could theoretically also remove
 	 * top-level plain JOIN/ON clauses, but it's probably not worth the
@@ -1180,6 +1186,7 @@ convert_EXISTS_sublink_to_join(PlannerInfo *root, SubLink *sublink,
 	/* Identify all the rels syntactically within the subselect */
 	subselect_varnos = get_relids_in_jointree((Node *) subselect->jointree,
 											  true);
+	Assert(!bms_is_empty(subselect_varnos));
 	Assert(bms_is_subset(right_varnos, subselect_varnos));
 
 	/* Now we can attach the modified subquery rtable to the parent */