diff --git a/src/strmanip.cpp b/src/strmanip.cpp
index 300c60e..f14b4f2 100644
--- a/src/strmanip.cpp
+++ b/src/strmanip.cpp
@@ -1,3 +1,4 @@
+#include <cctype>
 #include "../include/strmanip.h"
 
 bool Truth(std::string str)