mvc urlHelper mock for test

Question

I set up UrlHelper like this

    mockRepository = new MockRepository(MockBehavior.Default);
    _context = mockRepository.Create<HttpContextBase>();

    _requestBase = mockRepository.Create<HttpRequestBase>();
    _responseBase = mockRepository.Create<HttpResponseBase>();
        RouteCollection routes = new RouteCollection();
        routes.MapRoute(
            "auctioneersByLetter",
            "{culture}/auctioneers/{LetterFilter}",
            new { controller = "Client", action = "Index" },
            new { LetterFilter = @"^([a-zA-Z]{1}|all|0-9)$" }
        );

        routes.MapRoute(
            "auctioneersByLetter-search-filter",
            "{culture}/auctioneers/{LetterFilter}/search-filter",
            new { controller = "Client", action = "Index" },
            new { LetterFilter = @"^([a-zA-Z]{1}|all|0-9)$" }
        );

        _requestBase.SetupGet(x => x.Path).Returns("https://domain/en-gb/auctioneers");
        _requestBase.SetupGet(x => x.ApplicationPath).Returns("https://domain/en-gb/auctioneers");


        _context.SetupGet(c => c.Request).Returns(_requestBase.Object);
        _context.SetupGet(c => c.Response).Returns(_responseBase.Object);

        var routeData = new RouteData();
        routeData.Values.Add("LetterFilter", "");

        var requestContext = new RequestContext(_context.Object, routeData);
        _urlHelper = new UrlHelper(requestContext, routes);

but when I write test and call this code (passing test version of urlHelper) i gives me null

return urlHelper.RouteUrl("auctioneersByLetter", new
            {
                culture = Thread.CurrentThread.CurrentCulture,
                LetterFilter = value
            });