diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index a3a82ec1234..8243802199d 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -3539,7 +3539,13 @@ prepare_sort_from_pathkeys(PlannerInfo *root, Plan *lefttree, List *pathkeys,
 
 			if (!tle)
 			{
-				/* No matching tlist item; look for a computable expression */
+				/*
+				 * No matching tlist item; look for a computable expression.
+				 * Note that we treat Aggrefs as if they were variables; this
+				 * is necessary when attempting to sort the output from an Agg
+				 * node for use in a WindowFunc (since grouping_planner will
+				 * have treated the Aggrefs as variables, too).
+				 */
 				Expr	   *sortexpr = NULL;
 
 				foreach(j, ec->ec_members)
@@ -3552,7 +3558,7 @@ prepare_sort_from_pathkeys(PlannerInfo *root, Plan *lefttree, List *pathkeys,
 						continue;
 					sortexpr = em->em_expr;
 					exprvars = pull_var_clause((Node *) sortexpr,
-											   PVC_RECURSE_AGGREGATES,
+											   PVC_INCLUDE_AGGREGATES,
 											   PVC_INCLUDE_PLACEHOLDERS);
 					foreach(k, exprvars)
 					{
diff --git a/src/test/regress/expected/window.out b/src/test/regress/expected/window.out
index e42ce174381..048d463533a 100644
--- a/src/test/regress/expected/window.out
+++ b/src/test/regress/expected/window.out
@@ -594,6 +594,26 @@ SELECT SUM(COUNT(f1)) OVER () FROM int4_tbl WHERE f1=42;
    0
 (1 row)
 
+-- window function with ORDER BY an expression involving aggregates (9.1 bug)
+select ten,
+  sum(unique1) + sum(unique2) as res,
+  rank() over (order by sum(unique1) + sum(unique2)) as rank
+from tenk1
+group by ten order by ten;
+ ten |   res    | rank 
+-----+----------+------
+   0 |  9976146 |    4
+   1 | 10114187 |    9
+   2 | 10059554 |    8
+   3 |  9878541 |    1
+   4 |  9881005 |    2
+   5 |  9981670 |    5
+   6 |  9947099 |    3
+   7 | 10120309 |   10
+   8 |  9991305 |    6
+   9 | 10040184 |    7
+(10 rows)
+
 -- test non-default frame specifications
 SELECT four, ten,
 	sum(ten) over (partition by four order by ten),
diff --git a/src/test/regress/sql/window.sql b/src/test/regress/sql/window.sql
index 61da23a4a35..268430a260b 100644
--- a/src/test/regress/sql/window.sql
+++ b/src/test/regress/sql/window.sql
@@ -138,6 +138,13 @@ SELECT empno, depname, salary, bonus, depadj, MIN(bonus) OVER (ORDER BY empno),
 -- window function over ungrouped agg over empty row set (bug before 9.1)
 SELECT SUM(COUNT(f1)) OVER () FROM int4_tbl WHERE f1=42;
 
+-- window function with ORDER BY an expression involving aggregates (9.1 bug)
+select ten,
+  sum(unique1) + sum(unique2) as res,
+  rank() over (order by sum(unique1) + sum(unique2)) as rank
+from tenk1
+group by ten order by ten;
+
 -- test non-default frame specifications
 SELECT four, ten,
 	sum(ten) over (partition by four order by ten),