Syntax Error using dynamic linq with MVC 2.0's Ajax.BeginForm

Question

Hey Guys, I got some problem with dynamic linq (link) and Ajax.BeginForm. Serverside I got the following Controller Action:

[HttpPost]
public ActionResult Ajax(String filter)
{
    var count = (from e in context.Elements
                 where(filter)
                 select e).Count();

    return Json(new { ElementCount = count });
}

I get an exception "Syntax Error". Following the method which throws the exception from dynamic.cs

void ValidateToken(TokenId t, string errorMessage)
{
    if (token.id != t) throw ParseError(errorMessage);
}

Now the strangest thing is, the same Action returns without any errors when I call it via Html.BeginForm - a non-ajax request. The following works when called via a non-ajax form.

[HttpPost]
public ActionResult NonAjax(String filter)
{
    var count = (from e in context.Elements
                 where(filter)
                 select e).Count();

    ViewData["ElementCount"] = count;
    return View();
}

Has anyone encountered something like this - and maybe even solved it?