svn merge remote server versions to local workspace

Tried something seemingly simple that did not work as expected.
$ svn merge <remote>@N <remote>@M .

According to the svnbook the result should be getting the difference
between N and M and applying it to .
But it fails with a message about mixing access methods .... huh?

In fact $ svn diff <remote>@N <remote>@M does indeed produce a patch
with difference between N and M.

So why should the type of the local workspace matter?

--
Jerry Pendergraft
jerry.pendergraft@parvenu.com