Draftsight - cannot radius a corner - error "cannot fillet and entity with itself"

I really love Draftsight but cannot figure where I am going wrong with the radius command. I just get an error "cannot fillet an entity with itself", even before I've selected the second entity. I have tried everything. If two perpendicular  lines intersect at a corner, why is it so hard to turn the corner into a radiused one?! Thanks for your help. Chris