Bolt and Gervais tilesets in NPP

Hi

I am just doing some work on the tiles support for Ironband, which is
based off NPP.

When I switch to graphics mode, I find that it displays correctly with
'old' tiles, but if I go to 'David Bolt' or 'David Gervais' tiles,
then, while monsters and objects look fine, walls and other terrain
features show up as ASCII characters rather than tiles.

Can anyone comment on why this happens? Is it the intended behaviour
in NPP, or does it occur for some other reason?

Thanks
A.