Problem with git log -p

[sinelaw]

$ git log -p origin/master..origin/my_branch | fancydiff stdin
fancydiff: Could not open repository "."
CallStack (from HasCallStack):
  error, called at ./Git/Libgit2.hs:1545:21 in gitlib-libgit2-3.1.1.1-DHYqEGvvMZp3g1dyPQ2lvH:Git.Libgit2