Lattice: Drawing a single contour line with a negative value fails

Hi everybody,

 I want to add a single contourline to a levelplot. While everything works
fine if the value at which the line should be drawn is positive, there is an
error if the value is negative:

 library(lattice)
 my.panel <- function(..., at, contour=FALSE, labels=NULL) {
   panel.levelplot(..., at=at, contour=contour, labels=labels)
   panel.contourplot(..., contour=TRUE, labels="Contour", lwd=2,
      at=con)
 }

 x <- y <- 1:100
 df <- expand.grid(x=x, y=y)
 df$z <- apply(df, 1, function(x)
   (x[1]^2+x[2]^2)*ifelse(x[1]^2+x[2]^2 < 5000, -1,1))
 col.regions <- heat.colors(50)

 # Works
 con <- 1000
 levelplot(df$z~df$x+df$y, col.regions=col.regions, panel=my.panel)

 # Does not work
 con <- -1000
 levelplot(df$z~df$x+df$y, col.regions=col.regions, panel=my.panel)

 I've tracked down the error to the function "cut.default", which takes an
argument "breaks", which should be "either a numeric vector of two or more
cut points or a single number (greater than or equal to 2) giving the number
of intervals into which 'x' is to be cut."

 So it seems to me that a single contourline at a positive value works
because the value is interpreted as the number of intervals, even though
there is just this single line in the resulting plot.

 What would be the correct way to add just a single contour line to a
levelplot, or is it indeed a bug?