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 I have a gadget snapshot for which the standard internal energy block has been replaced by one containing the temperature.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