the only thing i could think of would be in the
modify convert polygons to subdiv pop up menu,
you might have some parameters that are not
correct. not sure if that is the problem and which
parameters cause i'm not looking at maya right
now. when that error happens, immediately go
to script editor (i think that's where you go)...
it sometimes would give you a lengthy
explanation to what is wrong if you are doing
something wrong, and it's not a bug...
shrugs
hope that helps...