Workflow Search for Object Filter

Have created a scheduled Workflow that searches for accounts that have the employeetype = contractor, and accountexpires > 90 days or accountexpires is empty.  When the workflow is run, it successfully finds the accounts that have accountexpires > 90 days, but not the ones that do not have an accountexpires set.

The filter is this.  (fyi, tried the first grouping both as 'AND' and 'OR' with the same results)