plot in win.metafile in nlme

Hallo!

I want to plot grouped data in a wmf-file. The
following example gives an error:

library(lattice)
library(nlme)
data(Loblolly) # example data from nlme
win.metafile("Loblolly.wmf")
plot(Loblolly)
dev.off()
 
After the plot(Loblolly) the following error occurs:
"Error in get(x, envir, mode, inherits) : variable
"win.metafile:Loblolly.wmf" was not found"
The file Loblolly.wmf exists but is just white.

With "normal" plot it works, e.g.:
win.metafile("Loblolly2.wmf")
plot(Loblolly$age, Loblolly$height)
dev.off()

Does anybody know how to solve the problem above or a
nice workaround?
(Using plotSave works in general but the colors are
bad.)

Karl