* 16 bytes.
*
* \param ctx GCM context
- * \param tag buffer for holding the tag (may be NULL if tag_len is 0)
- * \param tag_len length of the tag to generate
+ * \param tag buffer for holding the tag
+ * \param tag_len length of the tag to generate (must be at least 4)
*
* \return 0 if successful or MBEDTLS_ERR_GCM_BAD_INPUT
*/