Proximity search behaviour when a zipcode does not exists

If I search for a valid post code then the proximity search behaviour works as expected.

However, if I search for an invalid post code (one which does not exist in the zipcodes table), then I would expect no results to be displayed in the view. The behaviour I am seeing is that all results are displayed in the table as the location filter is not being applied.

Would it be possible to add validation on the zipcode entered into the filter?