XnView Software

When i use 'nconvert.exe -out ppm -o temp.ppm temp.png' there comes the error:

Error: This picture cannot be written using this format (temp.ppm)

The png file info from nconvert:
nconvert.exe -o temp.ppm -out ppm -info temp.png

temp.png : Success
Format : Portable Network Graphics
Name : png ...