How to update a tree-less bound branch?

[Stefan Monnier]

   % bzr update
   bzr: ERROR: No WorkingTree exists for "file://<blablabla>/.bzr/checkout/".
   % 

So I end up using "bzr pull" rather than "bzr update".  But last
I heard, the fact that "bzr pull" does the right thing in a bound
branch was considered as a lucky accident (more or less), so what is the
recommended way to do it?
Or is the above ERROR really just a bug?


        Stefan