Deleting an employee work schedule

Hi,

We are using employee work schedules with our time entry to pre-populate hours for some employees.  From time to time employees move to a different work group and no longer require a schedule.  We have always been able to delete the employee work schedule from the calendar maintenance screen so the employee no longer has that and no hours are created.  This time we are getting an error message when we try to delete it:

Any suggestions on what we are doing wrong or what needs to be done before we can delete this?  I'm not sure what the "parent" record is that the error message is talking about.