Why do I get a 'value required' error when I call an empty optional WebAPI parameter?

I have a WebAPI controller (TasksController) with a GET method like:

public IEnumerable<TimeUnitModel> Get(DateTime startDate, DateTime endDate, string projectCode = "")

If I call:

/api/tasks?startDate=2012%2F12%2F08&endDate=2012%2F12%2F15

then the correct result is returned.

If I call:

/api/tasks?startDate=2012%2F12%2F08&endDate=2012%2F12%2F15&projectCode=

then I get:

{"projectCode.String":"A value is required but was not present in the request."}

Here's what I have in the route config:

config.Routes.MapHttpRoute(
    name: "tasks_get",
    routeTemplate: "api/tasks",
    defaults: new { controller = "tasks", projectCode = RouteParameter.Optional}
);

Any idea why this happens?