Mailman CGI error!!!

The Mailman CGI wrapper encountered a fatal error. This entry is being stored in your syslog:
Failure to find group name for GID 2000.  Mailman
expected the CGI wrapper to be executed as group
"www-data", but the system's web server executed the
wrapper as GID 2000 for which the name could not be
found.  Try adding GID 2000 to your system as "www-data",
or tweak your web server to run the wrapper as group