Why isn't Null a constant in Blitz?

As the title says. I need to be able to pass an optional type handle to a function, like:

Function do_stuff(x, y, parent.myType=Null)

I just get an error message basically saying that Null isn't a constant. This little gem has really screwed up my plans. :/

Anyone know why Null isn't a constant in blitz, then? Beats me.