Vendor Change address

Hi All,

In vendor change address(xk02), the field values of "PO Box " & " Postal Code" field were 680 and 15106 respectivly. Now when i am just editing the field value of "PO Box " from 680 to Null (blank) & saving, i am getting the error "Specify PO Box for PO Box Postal Code". I am not changing " Postal Code" field value which is 15106. Need to know why is this error is coming.

Thanks