Git diff between branches not working in bitbucket pipelines

For my project I need files that were modified in a branch compared to master. I tried using 'git diff --name-only HEAD..master', but the following error is thrown.

fatal: ambiguous argument 'HEAD..master': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] – [<file>...]'

I am using ssh keys to fetch all the tags and I can see master tag being fetched. I would like to know is anything more needs to be done from my side or is this a bug.