weird behavior of string.Format method [duplicate]

This question already has an answer here:

• How string.Format handles null values? 4 answers

Why am I getting runtime exception at line

 Console.WriteLine(string.Format("Hello World {0}", null)); //Runtime 

while line

Console.WriteLine(string.Format("Hello World {0}", a));

works fine even though string a is null?

using System;

public class Program
{
    public static void Main()
    {
        string a = null;

        Console.WriteLine(string.Format("Hello World {0}", a));
        Console.WriteLine(string.Format("Hello World {0}", null)); //Runtime exception! Why?
    }
}

Here is DotNetFiddle