diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index e37a09e..63dad3d 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -390,7 +390,7 @@
 #endif /* MBEDTLS_SSL_PROTO_TLS1_2 */
 
     void (*update_checksum)(mbedtls_ssl_context *, const unsigned char *, size_t);
-    void (*calc_verify)(const mbedtls_ssl_context *, unsigned char *);
+    void (*calc_verify)(const mbedtls_ssl_context *, unsigned char *, size_t *);
     void (*calc_finished)(mbedtls_ssl_context *, unsigned char *, int);
     int  (*tls_prf)(const unsigned char *, size_t, const char *,
                     const unsigned char *, size_t,