# 30121: Fallback URL is missing

Log Type: APPLICATION

Log Level: ERROR

## Description

Twilio returns this error when the Link Shortening domain configuration does not include a `FallbackUrl`. Link Shortening domain defaults are set on the domain, and Twilio applies them to any Messaging Service that uses that domain.

### Possible causes

* You left the Domain fallback URL field blank in the Console when configuring Link Shortening for the domain.
* You sent a `POST` request to the Link Shortening domain's Config subresource without the `FallbackUrl` parameter.
* You associated a Messaging Service with a Link Shortening domain, but you did not complete the domain default configuration.

### Possible solutions

* Open the Link Shortening domain configuration in the Console, then fill in Domain fallback URL and save the changes.
* If you use the API, send `FallbackUrl` in a `POST` request to the Link Shortening domain's Config subresource.
* If you also need click tracking, provide `CallbackUrl` in the same request so the domain default configuration is complete.

#### Additional resources

* [Link Shortening Onboarding Guide](/docs/messaging/features/link-shortening/onboarding-guide)
* [Messaging Service resource](/docs/messaging/api/service-resource)