diff --git a/include/mbedtls/ssl_cookie.h b/include/mbedtls/ssl_cookie.h
index 71c258e..0f211e6 100644
--- a/include/mbedtls/ssl_cookie.h
+++ b/include/mbedtls/ssl_cookie.h
@@ -70,7 +70,7 @@
  * \brief          Setup cookie context (generate keys)
  */
 int mbedtls_ssl_cookie_setup(mbedtls_ssl_cookie_ctx *ctx,
-                             int (*f_rng)(void *, unsigned char *, size_t),
+                             mbedtls_f_rng_t *f_rng,
                              void *p_rng);
 
 /**