I'm not sure whether i'm doing something wrong or it is a bug. I'm running vips vips-8.9.1-Sun on aarch64 ubuntu. I'm trying to compress tiff image using ccittfax4 algorithm, which requires the input image to be black & white. Here what i'm doing:

vips pdfload input.pdf output1.v --n -1
vips colourspace output1.v output2.v b-w
vips tiffsave output2.v result.tiff --compression ccittfax4

on third command i'm getting:

Fax3SetupState: Bits/sample must be 1 for Group 3/4 encoding/decoding
wbuffer_write: write failed
unix error: Unknown error -1

Which is unexpected for me because the input image is black & white. Any help?