Tests involving TryValidateModel fail when run with test using custom HtmlHelper

Question

I have a test of an HtmlHelper extension method that works great when it is the only test that I run. It appears below. All of my other unit tests succeed when I run them, as long as I don't run this test at the same time.

But when I run this and all of my other unit tests together, all those that test a procedure that calls System.Web.Mvc.Controller.TryValidateModel fail on that line (regardless of the model) with the following exception:

System.InvalidCastException : Unable to cast object of type 'Castle.Proxies.INicheUserProxy' to type 'System.Web.Mvc.ModelMetadataProvider'.

All tests that don't call TryValidateModel succeed -- except for the test below, which fails with that same exception on the procedure's ModelMetadata.FromLambdaExpression line (see below).

I recognize that ModelMetaData ...

"Provides a container for common metadata, for the ModelMetadataProvider class, and for the ModelValidator class for a data model." https://msdn.microsoft.com/en-us/library/system.web.mvc.modelmetadata(v=vs.118).aspx

... and the procedure being tested starts like this ...

public static class HtmlHelperExtension
{
    public static MvcHtmlString DisplayForTruncated<TModel, TValue>(this HtmlHelper<TModel> html,
                                                                    Expression<Func<TModel, TValue>> expression)
    {
        var metaData = ModelMetadata.FromLambdaExpression(expression, html.ViewData);
        var rawInputText = metaData.SimpleDisplayText;
        // ....
    }
}

... so I'm fearful that by messing with the HtmlHelper in this test, I am effecting (ruining?) the ModelMetaData for every other test.

What am I doing that's causing those tests to fail when they are part of the same test run (but succeed when they're not)? And how do I correct it?

[TestFixture]
public class HtmlHelperExtensionTests
{
    [Test]
    public void DisplayForTruncated_happy_path()
    {
        const string summary = "AAA BBB CCC";
        var htmlHelper = DisplayForTruncated_BuildHtmlHelper(summary);

        var output = htmlHelper.DisplayForTruncated(m => m.FirstOrDefault().RearrangedSummary, NoLimit);

        Assert.IsNotNull(output);
        Assert.AreEqual(Utility.RearrangePersonSummary(summary), output.ToString());
    }

    private static HtmlHelper<Webdiyer.WebControls.Mvc.PagedList<PersonSummaryViewModel>>
        DisplayForTruncated_BuildHtmlHelper(string summary)
    {
        // from https://stackoverflow.com/questions/20498036/unit-testing-generic-htmlhelper-methods-with-nunit

        var viewModel = new PersonSummaryViewModel {Summary = summary};
        var listViewModel = new List<PersonSummaryViewModel> {viewModel};
        var inputPagedListModel = new Webdiyer.WebControls.Mvc.PagedList<PersonSummaryViewModel>
            (listViewModel, listViewModel.Count, 100);

        var viewData = new ViewDataDictionary(inputPagedListModel);
        var viewContext = new Mock<ViewContext>();
        viewContext.Setup(x => x.ViewData).Returns(viewData);

        var mockViewDataContainer = new Mock<IViewDataContainer>();
        mockViewDataContainer.Setup(v => v.ViewData).Returns(viewData);

        return new HtmlHelper<Webdiyer.WebControls.Mvc.PagedList<PersonSummaryViewModel>>
            (viewContext.Object, mockViewDataContainer.Object);
    }
}