ArgumentNull exception for args with String.Format()

gistfile1.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Text;
using System.Threading.Tasks;
using RazorEngine;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            int? val = null;
            Console.WriteLine("Value is: " + String.Format("{0:N4}", val));
            string value = string.Empty;

            try
            {
                // now with razor
                string template = "Hello @string.Format(\"{0:N4}\", Model.Value)! Welcome to Razor!";
                string result = Razor.Parse(template, new { Value = val });
                Console.WriteLine("razor output is: " + result);

            }
            catch (Exception e)
            {
                Console.WriteLine("Razor exception " + e.Message);
            }

            try
            {
                // now with razor and workaround
                string template = "Hello @(string.Format(\"{0:N4}{1}\", Model.Value, string.Empty))! Welcome to Razor!";
                string result = Razor.Parse(template, new { Value = val });
                Console.WriteLine("razor with workaround output is: " + result);

            }
            catch (Exception e)
            {
                Console.WriteLine("Razor exception " + e.Message);
            }

            try
            {
                Type t = typeof(string);
                MethodInfo info = t.GetMethod("Format", new[] { t, typeof(Object[]) });
                string output = (string)info.Invoke(value, new object[] { "{0:N4}", val });
                Console.WriteLine("Value is: " + output);
            }
            catch (TargetInvocationException e)
            {
                Console.WriteLine("String.Format(format, Object[] = null) exception " + e.InnerException.Message);
            }

            Console.ReadLine();
        }
    }
}