proximity filter in views returns no results

I've created cck nodes, with location info assigned. There are several nodes with zip codes entered. I've uploaded the zipcode database. I created a view with a proximity search filter.

As long as the filter is set to return any results, they show up. But if you enter a zip code in the filter and attempt to search by proximity, no results are returned, not even for nodes that have exactly that zip code.

It's the same whether the filter is exposed or not, if that makes any difference.

Is there a way to get this working?