MVC - Moq Unit Test FileContentResult (ActionResult) - NullRefernceException

So I'm posting to an MVC controller, which makes a call to a repository to get a Telerik report, then exports a PDF. I'm having trouble unit testing this and keep getting an error - System.NullReferenceException: Object reference not set to an instance of an object.

Controller

public class ReportController : Controller
{
    private IPDFRepository _pdfRepository;

    //Dependency Injection using Unity.MVC5 NuGet Package
    public ReportController(IPDFRepository pdfRepository)
    {
        _pdfRepository = pdfRepository;
    }

    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult PDFExport(PDFViewModel model)
    {
        byte[] report = _pdfRepository.BuildExport(model)
        return File(report, "application/pdf", model.SelectedReport + ".pdf");
    }
}

Unit Test

[TestMethod]
public void Report_PDFExport_Returns_ActionResult()
{
    //Arrange
    var mockRepository = new Mock<IPDFRepository>();
    mockRepository.Setup(x => x.BuildExport(It.IsAny<PDFViewModel>()));

    ReportController controller = new ReportController(mockRepository.Object);

    //Act
    ActionResult result = controller.PDFExport(It.IsAny<PDFViewModel>());

    //Assert
    Assert.IsInstanceOfType(result, typeof(ActionResult));
}

Now, I realize this has something to do with this return portion of my controller.

return File(report, "application/pdf", model.SelectedReport + ".pdf");

I can change that around to return string, test again and get this to work.

Also, if I comment out these last two lines of the unit test,

//Act
//ActionResult result = controller.PDFExport(It.IsAny<PDFViewModel>());

//Assert
//Assert.IsInstanceOfType(result, typeof(ActionResult));

it will run without erroring out. I can't figure out how to get around the null reference.