diff --git a/src/openalpr/utility.cpp b/src/openalpr/utility.cpp
index 7aea759..9932b13 100644
--- a/src/openalpr/utility.cpp
+++ b/src/openalpr/utility.cpp
@@ -19,6 +19,7 @@
 
 #include <opencv2/core/core.hpp>
 #include <functional>
+#include <cctype>
 
 #include "utility.h"