"fatal: ambiguous argument" when running --pull command

[npburns224]

When running sudo /usr/local/bin/pihole-cloudsync/pihole-cloudsync --pull I get the error

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

The error seems harmless as my lists are successfully pulled from my repo and gravity is updated as expected. I'm fairly inexperienced with using Git, but I'm fairly sure I've configured everything according to the instructions outlines in the readme.