`Git difftool -y` reports errors about newly created files

[SevereOverfl0w]

I have a commit which introduces a new file. When running :Git difftool -y HEAD^ the following message is output for each newly created file in the commit:

fatal: path 'path/to/the/file' exists on disk, but not in 'shashashashashashashasha'

The tabs are still created, but the error message is mildly annoying.