[yt-users] Problem with Units in a Derived Field

Hello! I have a gadget snapshot for which the standard internal energy block has been replaced by one containing the temperature. I'm trying to calculate a derived field for the neutral hydrogen number density, but I'm getting an error when I try to do ds.add_field(): http://paste.yt-project.org/show/5833/ However, I get an error about being unable to add quantities with differing units: http://paste.yt-project.org/show/5834/ The docs say not to do any unit conversion because that is apparently taken care of behind the scenes, so I didn't. I was just wondering if there was a way around this? Thanks! -Jared