# Error 21: Invalid URL

PayMe's message with `language: "en"`: **Please verify URL validity**

PayMe refused a URL in the request, and the whole call fails. `generate-sale` creates no sale. `status_additional_info` holds the URL.

## Usual causes

- A local address in `sale_callback_url` or `sale_return_url`. In the sandbox, `localhost` in either field and a private address both failed with HTTP 500.
- A malformed address, or one with two schemes.

## What to do

- When an address is local, leave the field out and log that you did. PayMe then uses the account's configured URL.
- To receive callbacks in development, use a tunnel. See [Config and Transport](/agents/steps/config-and-transport).

The full list of codes is on [Errors](/payments/getting-started/errors). If something else is going on, see [Troubleshooting by Symptom](/agents/errors/troubleshooting).