Errno::EPERM on gid

Process.fork do
  # drop privileges to uid/gid
  Process.initgroups(u.name, gid)
  Process::UID.change_privilege(u.uid)
  Process::GID.change_privilege(gid) # HERE we have Errno::EPERM
  puts "In BLOCK with uid: #{u.uid} and group #{gid}"
  # Invoke the caller's block of code.
  block.call #(user)
end


util.rb:54:in `change_privilege': Operation not permitted (Errno::EPERM)
   from util.rb:54:in `block in blk_as_user'
   from util.rb:49:in `fork'
   from util.rb:49:in `blk_as_user'
   from config.rb:288:in `write_rpmmacros'
   from util.rb:294:in `base_setup'
   from cli.rb:262:in `build_manager'
   from cli.rb:97:in `run'