Contour lines on 3D perspectives (grdview)

I’m using “fig.grdview” to plot a 3D perspective view of some bathymetry:

fig.grdview(grid=grid, region=region, projection=projection,
perspective=perspective, zscale=zscale,
cmap=True, surftype=‘s’)

I’d like to add contours to this, so tried:

fig.grdview(grid=grid, region=region, projection=projection,
perspective=perspective, zscale=zscale,
contourpen=‘0.1p,black’,
cmap=True,surftype=‘s’)

… but I can’t see how to control the contour interval.

Any tips welcome!