Intermittent nlobjSearchFilter error...

We have a script that automatically creates a task each time an email is sent-- (it records the template used so we can't simply use the 'message' records).

Anyway, I keep getting the error:

Error: SSS_INVALID_SRCH_OPERATOR
An nlobjSearchFilter contains an invalid operator, or is not in proper syntax: company.


The error comes from this code:

newfilters[1] = new nlobjSearchFilter('company',null,'anyof',messageentity);

var newsearchresults = nlapiSearchRecord('contact', null, newfilters, null); //CREATING A Search RESULT OBJECT

I seem to still get the error when I use nlobjSearchFilter('company',null,'is',messageentity);
and the error seems to be intermittent (only a few times a day when many emails are being sent. Any ideas what could be going on?