MVC :: Ajax.BeginForm: OnFailure?

I'm using Ajax.Begin form with MVC validation. When I'm in the server code and there's a validation error then ModelState.IsValid is false and that's good.If I have OnSuccess defined in my AjaxOptions why does my OnSuccess function get called when the ModelState is not valid?In my parital view:

using (Ajax.BeginForm("ProfileEdit",
new { controller =
"Account", action =
"ProfileEdit", id =
ViewData["ProfileID"]
}, new
AjaxOptions { UpdateTargetId
= "profileEdit", OnSuccess
= "OnSuccess" }))

In my controller I always just return PartialView();

I just want to display a success message in the view but OnSuccess gets called even if there is a modelstate error. If I try to throw a HttpException then I lose my ValidationSummary and other error messages.

To work around, I'm setting a ViewData["Success"] and displaying the message div in the .ascx if ViewData["Success"] != null.