custom panel help in lattice

Hi,

I am having an issue with a custom panel for lattice.  The problem
comes when I try passing a groups argument.

Here is the custom panel, a wrapper for smooth spline.  I copied
panel.loess and replaced the loess arguments with smooth.spline().
[Note: I would like to use the cross-validation fitting properties of
smooth.spline.]

library(lattice)

panel.smooth.spline<-function(x,y,w=NULL, df, spar = NULL, cv = FALSE,
       lwd=plot.line$lwd, lty=plot.line$lty,col, col.line=plot.line$col,
       type, horizontal=FALSE,... ){

           x <- as.numeric(x)
           y <- as.numeric(y)
           ok <- is.finite(x) & is.finite(y)
           if (sum(ok) < 1)
               return()
           if (!missing(col)) {
               if (missing(col.line))
                   col.line <- col
           }
           plot.line <- trellis.par.get("plot.line")
           if (horizontal) {
               spline <- smooth.spline(y[ok], x[ok], ...)
               panel.lines(x = spline$y, y = spline$x, col = col.line,
                   lty = lty, lwd = lwd, ...)
           }
           else {
               spline <- smooth.spline(x[ok], y[ok],...)
               panel.lines(x = spline$x, y = spline$y, col = col.line,
                   lty = lty, lwd = lwd, ...)
           }
       }


# Here is my test data frame
set.seed(25)
test<-data.frame(x=c(1:200), y=rnorm(200), groups=gl(4,200/4))

# This call to xyplot works, but the smoother colors are not unique.

xyplot(y~x|groups, data=test,
       panel=function(...){
               panel.xyplot(...)
               panel.smooth.spline(...)
               })

# This call to xyplot doesn't work and results in an error "error
using packet"

xyplot(y~x|groups, data=test, groups=groups,
       panel=function(...){
               panel.xyplot(...)
               panel.smooth.spline(...)
               })

I think this should be quite simple but I must be too simple minded.
Thanks for any help.