Grid gpar(col=NULL)

I have found that gpar(col=NULL) doesn't behave as I expect it to. Is this
due to a misconception on my part, or a bug?

library(grid)
grid.newpage()
grid.circle(gp=gpar(fill="yellow",col=NULL))
# Draws a yellow circle with no border, as it should.
# But if you resize the graphics window,
# the circle acquires a black border.

I am running Rgui for R 1.7.0 on Windows XP.

Damon Wischik.