diff --git a/src/base/tools/String.cpp b/src/base/tools/String.cpp
index c7bc3d1b1..7778c6da6 100644
--- a/src/base/tools/String.cpp
+++ b/src/base/tools/String.cpp
@@ -23,6 +23,9 @@
  */
 
 
+#include <ctype.h>
+
+
 #include "base/tools/String.h"
 #include "rapidjson/document.h"