"gptzfsboot: No ZFS pools located, can't boot." - What am I doing wrong?

[Hubert Hauser]

I'm trying to do FreeBSD dual-boot with Debian on legacy BIOS. My
partition scheme is:

 - /dev/ada0 (gpt)
  - /dev/ada0p1 (bios boot)
  - /dev/ada0p2 (linux filesystem)
  - /dev/ada0p3 (linux filesystem)
  - /dev/ada0p4 (freebsd boot)
  - /dev/ada0p5 (freebsd swap)
  - /dev/ada0p6 (freebsd zfs)

I've followed these instructions (link:
https://wiki.freebsd.org/MasonLoringBliss/LegacyZFSandGELI) but
unfortunately, I've got the following error during booting system:
"gptzfsboot: No ZFS pools located, can't boot."

What am I doing wrong?