can't diffsplit

[Lev Lvovsky]

I have two files which I'd like to compare via diffsplit in an  
existing vim session - trying to do so by issuing ':diffsplit  
<filename>' when the other one is already in the window gets me the  
error:

E97: Cannot create diffs
I can properly open the diff in a separate instance via 'vimdiff', as  
well as ':diffsplit' - is there something wrong with my existing  
session?

thanks!
-lev