This bug is (at least in our case) caused by libblkid (through udevd/udevadm) reporting the disklabel type as "dos" while the os-prober script looks for "msdos". This was fixed upstream in Debian os-prober 1.76 as a fix for Debian bug #817023 for which I have attached the patch, though I recommend pulling the upstream changes directly.
This bug is (at least in our case) caused by libblkid (through udevd/udevadm) reporting the disklabel type as "dos" while the os-prober script looks for "msdos". This was fixed upstream in Debian os-prober 1.76 as a fix for Debian bug #817023 for which I have attached the patch, though I recommend pulling the upstream changes directly.