[Koha] error message

[Scott Owen]

 Software error:

{UNKNOWN}: Can't call method "unblessed" on an undefined value at
/usr/share/koha/lib/C4/Circulation.pm line 2181, <DATA> line 751. at
/usr/share/koha/lib/C4/Circulation.pm line 2201

For help, please send mail to the webmaster ([no address given]), giving
this error message and the time and date of the error.
*********************************************************************

one of my people reports getting this message "occasionally" .

And ideas ??

-S