retrieval of mergeinfo unsupported error

Question

I am running SVN 1.6.12 on an old Linux box. I am getting a 'retrieval of mergeinfo unsupported' error when trying to do a reintegrate merge.

I connect to the server using SVN+SSH:

ssh USER@HOST svnserve --version
svnserve, version 1.6.12 (r955767)
   compiled Sep 15 2010, 10:01:35

Copyright (C) 2000-2009 CollabNet.
Subversion is open source software, see http://subversion.tigris.org/
This product includes software developed by CollabNet (http://www.Collab.Net/).

The following repository back-end (FS) modules are available:

* fs_fs : Module for working with a plain file (FSFS) repository.

I ran 'svnadmin upgrade testrepo' on the repository. Here is the version information:

cat testrepo/db/format          
4
layout sharded 1000

Clearly at least SVN 1.5.

What am I doing wrong?

Thanks.