Overriding ToString and `%A` specifier

If a override ToString in a type

type TestMe ()=
    override __.ToString() = null

and then I output it through the "%A" specifier

printfn "*%A*" (TestMe())

why does it throw a System.NullReferenceException?

I would have expected it to behave like

printfn "*%A*" null

which simply prints <null> without any exceptions.