"Unknown special file or filesystem" during normal boot

[okeeblow]

Hi all, I have a FreeBSD 9.1-RELEASE amd64 system installed following this guide.

Here are the filesystems on my boot drive:

NAME                       USED  AVAIL  REFER  MOUNTPOINT
aloe                      6.14G  48.5G   388M  /
aloe/swap                 2.06G  50.6G    72K  -
aloe/tmp                   200K  48.5G   200K  /tmp
aloe/usr                  3.54G  48.5G   587M  /usr
aloe/usr/home              420K  48.5G   420K  /usr/home
aloe/usr/ports            2.47G  48.5G  2.25G  /usr/ports
aloe/usr/ports/distfiles   225M  48.5G   225M  /usr/ports/distfiles
aloe/usr/ports/packages    144K  48.5G   144K  /usr/ports/packages
aloe/usr/src               509M  48.5G   509M  /usr/src
aloe/var                   156M  48.5G  2.19M  /var
aloe/var/crash             148K  48.5G   148K  /var/crash
aloe/var/db                152M  48.5G   150M  /var/db
aloe/var/db/pkg           1.66M  48.5G  1.66M  /var/db/pkg
aloe/var/empty             144K  48.5G   144K  /var/empty
aloe/var/log               440K  48.5G   440K  /var/log
aloe/var/mail              144K  48.5G   144K  /var/mail
aloe/var/run               276K  48.5G   276K  /var/run
aloe/var/tmp               200K  48.5G   200K  /var/tmp

And my /boot/loader.conf:

zfs_load="YES"
vfs.root.mountfrom="zfs:aloe"

During boot the kernel fails to mount /usr with the error "unknown special file or filesystem". Boot continues, but nothing starts properly and I'm unable to get a shell due to inaccessible /usr/libexec/getty.

However, I can start the system without issue by booting to single user mode, executing

zfs mount -a

in the shell, then exiting to continue multi-user boot.

How can I make this fs mount correctly at boot without intervention?