diff --git a/library/ssl_cookie.c b/library/ssl_cookie.c
index 519d6d7..3781796 100644
--- a/library/ssl_cookie.c
+++ b/library/ssl_cookie.c
@@ -122,6 +122,7 @@
 /*
  * Generate the HMAC part of a cookie
  */
+MBEDTLS_CHECK_RETURN_CRITICAL
 static int ssl_cookie_hmac( mbedtls_md_context_t *hmac_ctx,
                             const unsigned char time[4],
                             unsigned char **p, unsigned char *end,