Why does C# allow you to 'throw null'?

While writing some particularly complex exception handling code, someone asked, don't you need to make sure that your exception object isn't null? And I said, of course not, but then decided to try it. Apparently, you can throw null, but it is still turned into an exception somewhere.

Why is this allowed?

throw null;

In this snippet, thankfully 'ex' is not null, but could it ever be?

try
{
  throw null;
}
catch (Exception ex)
{
  //can ex ever be null?

  //thankfully, it isn't null, but is
  //ex is System.NullReferenceException
}