git show on bare repository?

I could have sworn I used to be able to use git show to display the contents of a file from the server, but now it always says this (if I use -- then it doesn't show anything):

fatal: ambiguous argument 'master:file.txt': unknown revision or path not in the working tree. Use '--' to separate paths from revisions

What I'm trying to do is allow people to get a single file without having to clone the entirely repository, like this:

ssh git.example.com "cd /MyRepo.git && git show master:File.txt"

If I do this with a non-bare repository, it works. The server is running git version 1.6.3.2 (I'm running 1.6.4 locally where it works).