Using transparent contour fills in gsn_csm_...

Hi

I have a field with valid points everywhere that I want to contour fill
over a map (on a 2D lat/lon projection) that has land/sea filled colors
(via mpFillColors). I have specified some cnLevels and associated
cnFillColors, and would like at least one of the filled contours to be
transparent. I'm plotting using gsn_csm_contour_map with the HighRes
coastline. However, when I set one of the cnFillColors to -1, ncl responds

warning:LabelBarDraw: cannot use raster fill if explicit box sizing set
or any color index is transparent: defaulting to normal fill

At which point the hoped for transparent fill is overwritten by a
non-transparent color, and I lose the land/sea coloring

Thanks for your help

Michael

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