git diff shows no output

Hello,

I'm a new user of git, and I meet a very strange behaviour with the git diff command.
I have modified stuff in a file, with changes both in the index and the working tree.

"git diff" shows nothing, while "git diff | cat" shows the diff, as well as "git diff>output.txt" puts diff in output.txt.
Quite Weird

I'm on Mac OS X 10.6.4, using git 1.7.2.1, installed via macports.

Best,
-- 
Charles