HttpUtility.ParseQueryString in partial trust environment

I'm just getting started with writing an XBAP page. I need to pass some information into the XBAP context via query string parameters. I am expecting the XBAP to execute in a partial trust context.

When I run the following XBAP code-behind hosted on IIS, it throws the following

System.Reflection.TargetInvocationException:

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.

---> System.Security.SecurityException: That assembly does not allow partially trusted callers. at UI.UIEntryPage..ctor()

If I comment out the call to HttpUtility.ParseQueryString(), the XBAP loads properly, so it seems I'm not allowed to use HttpUtility to parse the query string in a partial trust context? (I find this surprising since a static method to parse a string into a NameValueCollection doesn't really seem like a security risk...)

Is there anything I can do to call this method in a partial trust context? Or, failing that, how can I parse the query string parameters securely without using HttpUtility?

namespace UI
{
    /// <summary>
    /// Interaction logic for UIEntryPage.xaml
    /// </summary>
    public partial class UIEntryPage : Page
    {
        public UIEntryPage()
        {
            InitializeComponent();

            if (!ApplicationDeployment.IsNetworkDeployed) return;

            Uri launchUri = ApplicationDeployment.CurrentDeployment.ActivationUri;

            string query = launchUri.Query;

            // This line causes an exception to be thrown
            // when the XBAP is loaded.
            NameValueCollection uriParameters = HttpUtility.ParseQueryString(query);

            foreach (string key in uriParameters.AllKeys)
            {
                UriParameters.Add(key, uriParameters[key]);
            }

            DataContext = this;
        }

        public readonly Dictionary<string, string> UriParameters = new Dictionary<string, string>();
        }
    }
}