diff --git a/library/ssl_cookie.c b/library/ssl_cookie.c
index 40b8913..5936d35 100644
--- a/library/ssl_cookie.c
+++ b/library/ssl_cookie.c
@@ -36,6 +36,7 @@
 #include "ssl_misc.h"
 #include "mbedtls/error.h"
 #include "mbedtls/platform_util.h"
+#include "constant_time.h"
 
 #include <string.h>