Combobox Validation Rule (1 Viewer)

I have 2 comboboxes (cboTeacher1 and cboTeacher2) that should not contain the same entry and in the Property dialog box have entered "<> cboTeacher2" in the Validation Rule field for cboTeacher1 and placed "You can not have the same teacher's name in both Primary and Secondary Teacher" in the Validation Text property field.

It was brought to my attention that although it works, but apparently if you delete the value in cboTeacher1 and press tab leaving the combobox empty, the validation rule is activated and "You can not have the same teacher's name in both Primary and Secondary Teacher" is displayed.

Since cboTeacher2 is not blank, and therefore cboTeacher1 and cboTeacher2 are different in this scenario, I would have thought that the validation rule would not have displayed the message. Why is it displaying the message and how do I get it to stop if someone empties the combobox?

This thing is actually working properly. The thing is that value has not been changed because you just cleared the combo box .... so even if you cleared the combo box and tries to leave the field it still has the same value as the first combo.