diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 923c671..c5ffa4d 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -39,6 +39,7 @@
 #include "mbedtls/error.h"
 #include "mbedtls/platform_util.h"
 #include "mbedtls/version.h"
+#include "constant_time.h"
 
 #include <string.h>