ASP.NET MVC - Unit testing RenderPartialViewToString() with Moq framework?

I'm using this helper method to turn my PartialViewResult into string and returning it as Json - http://www.atlanticbt.com/blog/asp-net-mvc-using-ajax-json-and-partialviews/

My problem is that I'm using Moq to mock the controller, and whenever I run unit test that uses this RenderPartialViewToString() method, I got the "Object reference not set to an instance of an object." error on ControllerContext.

private ProgramsController GetController()
{
var mockHttpContext = new Mock();
mockHttpContext.SetupGet(p => p.HttpContext.User.Identity.Name).Returns("test");
mockHttpContext.SetupGet(p => p.HttpContext.Request.IsAuthenticated).Returns(true);
// Mock Repositories
var mockOrganizationRepository = new MockOrganizationRepository(MockData.MockOrganizationsData());
var mockIRenderPartial = new BaseController();
var controller = new ProgramsController(mockOrganizationRepository, mockIRenderPartial);
controller.ControllerContext = mockHttpContext.Object;
return controller;
}

This returns a proxy controller, and maybe it's the reason why I got that error. Any idea how to unit testing this?

Thank you very much.