diff --git a/src/string_utils.h b/src/string_utils.h
index 748e8699..b7e14b21 100644
--- a/src/string_utils.h
+++ b/src/string_utils.h
@@ -7,6 +7,7 @@ extern "C" {
 
 #include <stdlib.h>
 #include <string.h>
+#include <ctype.h>
 #include <stdbool.h>
 #include <stdarg.h>
 #include "collections.h"