Why does AngularJS include an empty option in select?

I've been working with AngularJS for the last few weeks, and the one thing which is really bothering me is that even after trying all permutations or the configuration defined in the specification at http://docs.angularjs.org/api/ng.directive:select, I still get an empty option as the first child of select element. Here's the Jade: select.span9(ng-model='form.type', required, ng-options='option.va