Ack: Search directory tree for files with a particular extension

I basically just want to do ack foo *.citrus and have ack drill down and find the string 'foo' in all Citrus files in the current directory and below. The trouble is that this won't work if there aren't any Citrus files in the current directory.

I tried messing with -G without success. Do I really need to add a file type in .ackrc just to limit the search to files with a given extension?