diff --git a/Source/kwsys/CommandLineArguments.cxx b/Source/kwsys/CommandLineArguments.cxx
index 0eca02af2..d374ad645 100644
--- a/Source/kwsys/CommandLineArguments.cxx
+++ b/Source/kwsys/CommandLineArguments.cxx
@@ -23,6 +23,7 @@
 #include KWSYS_HEADER(ios/iostream)
 
 #include <stdio.h>
+#include <stdlib.h>
 
 namespace KWSYS_NAMESPACE
 {