Cannot create pull request between same branch in different repos

When trying to create a pull request between the same branch in repositories, there is an error message "You cannot merge a branch with itself".

Steps to reproduce:
1. View 'files' on fork repository
2. Switch branch to a branch other than 'master' e.g. 'test-branch'
3. Click the create pull request button -> This should bring you to the create pull request page with the source branch prefilled with 'test-branch'. Source repo should be your fork and target repo should be the origin.
4. Switch target branch to the same branch ('test-branch'). Error message should appear