MVC custom helper appending htmlAttributes

I have a custom helper class to highlight the selected menu item,

<li class="nav-item">
    @Html.MenuLink("Contact000", "Contact", "Home", new { @class = "nav-link" })
</li>   

I want to use the helper htmlattributes and append to CSS if selected condition is true. I have seen a Similiar question but was not able to find a question using

var attrs = HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes);

Here is my code

public static MvcHtmlString MenuLink(this HtmlHelper helper, string text, string action, string controller, object htmlAttributes)
{
    var routeData = helper.ViewContext.RouteData.Values;
    var currentController = routeData["controller"];
    var currentAction = routeData["action"];

    var attrs = HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes);

    if (String.Equals(action, currentAction as string,
        StringComparison.OrdinalIgnoreCase)
        &&
        String.Equals(controller, currentController as string,
        StringComparison.OrdinalIgnoreCase))
    {
        if (attrs.ContainsKey("class"))
        {
            attrs["class"] += " " + "currentMenuItem";
        }

        return helper.ActionLink(text, action, controller, attrs);
    }

    return helper.ActionLink(text, action, controller, htmlAttributes);
}

using

 var attrs = HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes);

I can append to the attributes.

when I return the helper.actionlink the attrs is not parsed correctly, below is the html generated,

<li class="nav-item">
    <a Count="1" Keys="System.Collections.Generic.Dictionary`2+KeyCollection[System.String,System.Object]" Values="System.Collections.Generic.Dictionary`2+ValueCollection[System.String,System.Object]" href="/Home/Contact?Length=4">Contact000</a>
</li>

I am trying to get the following,

<li class="nav-item">
    <a class="nav-link currentMenuItem" href="/Home/Contact">Contact000</a>
</li>

How do I convert attrs back into htmlAttributes for the helper to work ?