error trying to convert to xpm

[clapperboy]

Hello, I want to use nconvert to convert bmp to xpm.
here is the line I use,

nconvert.exe -out xpm test.bmp

I get this error message
Error: This picture cannot be written using this format <test.bmp>

I can convert the bmp to format like tiff and jpeg. I am wondering if there are any issue to the conversion to xpm.
or maybe convert it to some format first and then to xpm. I have browsed through the forums and the web and havent found anything so afr.

Thanks

[xnview]

XMP doesn't support 24bits, so add -colors 256