Sysupgrade - Image metadata not present on RUT955

sysupgrade -F -v -n -f /tmp/keep.tar.gz /tmp/fw.bin
Mon Jul 12 19:44:43 UTC 2021 upgrade: Image metadata not present
Image check failed but --force given - will update anyway!
Mon Jul 12 19:44:43 UTC 2021 upgrade: Commencing upgrade. Closing all shell sessions.

Need to use -F here, on my RUT955 running custom image.
However, simple
sysupgrade -v -n /tmp/fw.bin
always works.
/tmp/keep.tar.gz is about 4kb, contains one complete directory + 1 single file