diff --git a/src/StringUtil.cc b/src/StringUtil.cc
index 6d4a5826..22957840 100644
--- a/src/StringUtil.cc
+++ b/src/StringUtil.cc
@@ -24,6 +24,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
+#include <ctype.h>
 
 //------- strdup ------------------------
 //TODO: comment this