port selfupdate failling

[Uli Wienands]

Dear all,

I have been trying to update my MacPorts installation by running  
"sudo port -d selfupdate". It fails with the following message:

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===> making install in doc
/usr/bin/install -c -d -o root -g 501 -m 755 /opt/local
/usr/bin/install -c -d -o root -g 501 -m 755 /opt/local/etc/macports
cat prefix.mtree | /usr/sbin/mtree -U -e -p /opt/local > /dev/null
mtree: line 6: unknown group 501
make[1]: *** [install] Error 1
make: *** [install] Error 1

I gather form this that it wants to do something in or to group 501.  
Well, my acount is in 502, not 501. 501 does not exist, but why would  
it matter?? 502 (the group) has my own name. 80 is the group called  
admin. I have sudo privs (i.e. administrator account).

?? Any hint how to get beyond this? (And I do not want to change my  
group id even if this were possible. Too much risk.).

Thanks in advance

Mac Dude