"must be ancestrally related"

I am trying to take advantage of Subversion 1.5's new merge tracking
feature. I imported a project to (A):
http://artefactual.com/svn/qubit/trunk/symfony/vendor

- and then used "svn cp" to copy that project to (B):
http://artefactual.com/svn/qubit/trunk/qubit/lib/vendor/symfony

Now, I expect that running:

$ svn merge --reintegrate
http://artefactual.com/svn/qubit/trunk/symfony/vendor lib/vendor/symfony

- in a http://artefactual.com/svn/qubit/trunk/qubit working copy, should
merge all outstanding changes from A to B. Instead however, I get the
following error:

$ svn merge --reintegrate
http://artefactual.com/svn/qubit/trunk/symfony/vendor lib/vendor/symfony
svn: 'http://artefactual.com/svn/qubit/trunk/symfony/vendor@1428' must
be ancestrally related to
'http://artefactual.com/svn/qubit/trunk/qubit/lib/vendor/symfony@1428'
$

I thought that by using "svn cp" to copy A to B, B and A would be
ancestrally related? What is wrong?

One wrinkle is that the repository was created a year ago with
Subversion 1.4 "svnadmin create". The server was upgraded to Subversion
1.5 *before* I ran "svn cp", and I used a Subversion 1.5 client to "svn
cp". However I upgraded the repository to the Subversion 1.5 format
using "svnadmin upgrade" and "svn-populate-node-origins-index" *after* I
ran "svn cp". Could this be the problem?