This client is too old to work with working copy

Error message:

# svn status
svn: This client is too old to work with working copy 'rpc'. You need
to get a newer Subversion client, or to downgrade this working copy.
See http://subversion.tigris.org/faq.html#working-copy-format-change
for details.

How to resolve a problem:

# echo "deb http://www.backports.org/debian lenny-backports main contrib non-free" >> /etc/apt/sources.list
# wget -O - http://backports.org/debian/archive.key | apt-key add -
# apt-get update
# apt-get -t lenny-backports install subversion subversion-tools libapache2-svn