How do I stop all members being founds by submitting blank search form?

Hi,

I have an advanced search form with just two fields – Keyword and Post Code.

Designed to force visitors to nominate one Post Code to reveal in the search results only the members in that chosen Post Code.

But if you leave the two fields blank and simply click search – it reveals ALL members in the database.

How do I make the Post Code field compulsory – so it restricts every search to that Post Code?

Thanks