diff --git a/PExpr.cc b/PExpr.cc
index 8d1ca905b..8a32b0506 100644
--- a/PExpr.cc
+++ b/PExpr.cc
@@ -20,6 +20,7 @@
 
 # include "config.h"
 
+# include  <algorithm>
 # include  <iostream>
 
 # include  "compiler.h"