defmacro and bwplot incompatibilities?

bwplot is a trellis function. There is something very basic about the way that library works that any intro to trellis/lattice should tell you:

http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-do-lattice_002ftrellis-graphics-not-work_003f
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My macro doesn't work for bwplot. But is working elsewhere.
What changes should i make to fix my macro.
Thanks.

The complete code sample is:
library(gtools)
library(lattice)

pic <- defmacro(fn, plotfunc, expr={png(filename=fn); plotfunc;
dev.off()})

# this one fails.
pic("bw.png", {bwplot(Species ~ Sepal.Length, data=iris)})

# this one works
pic("p.png", {plot(iris$Sepal.Length)}

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