Failure is not returned by grub-mkrescue

[Wink Saville]

I'm using grub-mkrescue in a script and I wasn't seeing an error on bad
input. Below is a test showing no failure is reported even though the input
file, xx, does not exist:

$ grub-mkrescue -o xx.img xx
xorriso 1.4.2 : RockRidge filesystem manipulator, libburnia project.

Drive current: -outdev 'stdio:xx.img'
Media current: stdio file, overwriteable
Media status : is blank
Media summary: 0 sessions, 0 data blocks, 0 data,  421g free
Added to ISO image: directory '/'='/tmp/grub.vDHSgN'
xorriso : UPDATE : 875 files added in 1 seconds
xorriso : FAILURE : Cannot determine attributes of source file
'/home/wink/prgs/sadie/build-pc_x86_64/xx' : No such file or directory
xorriso : UPDATE : 875 files added in 1 seconds
xorriso : aborting : -abort_on 'FAILURE' encountered 'FAILURE'

$ echo $?
0


Should I file a bug or have I made some mistake?

-- Wink