error message: undefined method `parentof?'

Hi Puppet users, I occasionally see this error message in my logfiles,
and was wondering what it could possibly mean.  I found no mention
of it in the wiki or list archives, and so I turn to you.

Could not run Puppet::Network::Client::Master: undefined method `parentof?' for 
nil:NilClass

It appears in the log files at the end of a configuration run, like this:

Oct 17 20:13:30 domU-12-31-33-00-03-66 puppetd[2022]: Finished configuration 
run in 84.61 seconds
Oct 17 20:13:30 domU-12-31-33-00-03-66 puppetd[2022]: Could not run 
Puppet::Network::Client::Master: undefined method `parentof?' for nil:NilClass
Oct 17 20:14:03 domU-12-31-33-00-03-66 puppetd[2022]: Starting configuration run

During testing I am running the configuration every 30 seconds, and on two
occasions that I've observed this recently it was in a configuration run that
took longer than that.  However, this may be coincidence, because a long
configuration run is not sufficient to create the message.

Any ideas about what this means?  If it might be a problem in my configuration,
any suggestions about what to look for?

Thanks, Alex