Problem with port selfupdate on OS X 10.8.2

[xave@…]

With sudo port -v selfupdate I receive:

===>making install in doc
/usr/bin/install -c -d -o macports -g 502 -m 0755 /opt/local
/usr/bin/install -c -d -o macports -g 502 -m 0755 /opt/local/etc/macports
< prefix.mtree /usr/sbin/mtree -U -e -p /opt/local > /dev/null
mtree: line 6: unknown group 502
make[1]: *** [install] Error 1
make: *** [install] Error 1
Command failed: cd /opt/local/var/macports/sources/rsync.macports.org/release/tarballs/base && CC=/usr/bin/cc 
./configure --prefix=/opt/local --with-tclpackage=/Library/Tcl --with-install-user=macports --with-install-group=502 
--with-directory-mode=0755 --enable-readline && make && make install SELFUPDATING=1
Exit code: 2
Error: /opt/local/bin/port: port selfupdate failed: Error installing new MacPorts base: command execution failed