ASI_LU_Employee_Lookup_Scroll_Fill_After

We are getting the following error on a intermittent base when trying to lookup employees in Dynamics:

Illegal address fro field 'Employee ID' in script 'ASI_LU_Employee_Scroll_Fill+After' .

This occurs maybe twice a day. I have checked the upr00100 table for obvious issues in the Employee ID field but found none. Any suggesions would be greatly apprediated

Thanks
Chuck