[R] does grid.rect() not accept NULL argument from gpar(col=NULL) ?

[Mark Heckmann]

I want to draw a grid rectangle without a border.

?gpar says:

"Specifying the value NULL for a parameter is the same as not specifying any
value for that parameter, except for col and fill, where NULL indicates not
to draw a border or not to fill an area (respectively)." 

pushViewport(viewport(height=unit(.8, "npc")))
	grid.rect(gp=gpar(col=NULL, fill="green"))
popViewport()

Still a border is drawn. What am I doing wrong?

TIA, Mark