diff --git a/src/clazy_stl.h b/src/clazy_stl.h
--- a/src/clazy_stl.h
+++ b/src/clazy_stl.h
@@ -26,6 +26,7 @@
 #include <clang/AST/Stmt.h>

 #include <algorithm>
+#include <cctype>
 #include <sstream>

 namespace clazy {