Very Basic Angular Scenario Not Working

Can anyone tell me why this extremely basic example in Angular does not work?

<div ng-app>
    <div ng-controller="DemoCtrl">
        Hi, {{firstName}} {{lastName}}!
    </div>
</div>

function DemoCtrl($scope)
{
    $scope.firstName = "Steve";
    $scope.lastName = "Smith";
}

It gives the error "Argument 'DemoCtrl' is not a function, got undefined" . . . JsFiddle is here: http://jsfiddle.net/SfU3z/