border of a polygon in contour.kriging - geoR

Dear all,

When a conventional kriging and then a contour plot is limited with a polygon (as possible with krige.conv and contour.kriging), the
polygon border is displayed in black by default.

kc<-krige.conv(CZdata,loc=pred.grid,borders=czpoly,krige=krige.control(obj.m=ls))
plot(CZcoord,xlab="x",ylab="y",type="n",asp=1)
contour(kc,loc=pred.grid,add=T)

I would like to be capable to get the contour lines only (limited by the polygon, but without the polygon border displayed). I tried
to pass the argument border=NULL, unsuccessfully:

contour(kc,loc=pred.grid,add=T,border=NULL)

Error in contour(as.double(x), as.double(y), z, as.double(levels), labels,  :
        dimension mismatch
In addition: Warning message:
Replacement length not a multiple of the elements to replace in matrix(...)

Any hint?

Thanks in advance,

Patrick Giraudoux