Cognito: VerifyUserAttribute 'email' responds with InternalServerError when using another email already in the pool

In our Cognito instance we define the 'email' as the username. We implemented a workflow, that the user should be able to change the email-address. When the user takes an email address already used in the pool and a valid Code. We get in InternalServerError. (See: https://docs.aws.amazon.com/cognito-user-identity-pools/latest/APIReference/API_VerifyUserAttribute.html) My Expectation would be, that that there is an AliasExistsException. Can you verify, that this is a bug in Cognito?