String.Format and IFormatProvider

Spotted some odd behaviour with String.Format when there is more than one parameter and you are supplying a format string as a variable that could potentially be null.

For instance, I have:

private object FooBar()
{
      return null;

}


private void SomeMethod()
{
          var foo = FooBar() as string;
     var bar = String.Format(foo, "foo", "bar");
}

ReSharper offers to 'Check if 'Foo' is not null'
When I choose this option, I get:

private void SomeMethod()
{
    var foo = FooBar() as string;
    if (foo != null)
    {
         var bar = String.Format((IFormatProvider) foo, "foo", "bar");
    }

}
which clearly isn't right.

whereas if there is only a single parameter then resharper does it properly:

private void SomeMethod()
{
     var foo = FooBar() as string;
     if (foo != null)
     {
         var bar = String.Format(foo, "foo");
     }

}