You have turned on the ServerFilterByForm property. However, when users open the form and select from the combo boxes, the only choices are "Is Null" and "Is Not Null". How do you get the combo boxes to show a list of valid values for that field?
If you turn on the ServerFilterByForm property, your form will open in a special view that turns text boxes into combo boxes. This allows users to define their own server filters at runtime, which are then processed by SQL Server before the record source data is returned to the form. However, you'll often see only the values shown in Figure 14-8 when you expand one of the combo boxes.