diff --git a/lib/cookie.c b/lib/cookie.c
index 41a366d2da..913ffabebc 100644
--- a/lib/cookie.c
+++ b/lib/cookie.c
@@ -84,6 +84,7 @@ Example set of cookies:
 
 #include <stdlib.h>
 #include <string.h>
+#include <ctype.h>
 
 #define _MPRINTF_REPLACE /* without this on windows OS we get undefined reference to snprintf */
 #include <curl/mprintf.h>