Passing PointF as Optional Parameter

I want to pass PointF as optional Parameter. But I am finding it difficult to assign default value for PointF.
I can't assign Nothing. If I do it becomes (0,0) which can be working value hence can't be used as a unique default value.
I am not able to assign any other value. I tried (Optional P1 As PointF = New PointF(-100, -100) but it does not work.

Public MyFunc(Optional P1 As PointF = ????)

Please help.