Jail fails to unmount a directory

[io7m+org.freebsd.jail at io7m.com]

Hello.

I have an incredibly trivial jail setup:

/usr/jail/com.example.service0 is the root of the jail. 
/usr/jail/com.example.service0/base is an empty directory.
/usr/jail/base is a directory containing binaries.

I use the following jail configuration:

com_example_service0
{
  exec.start     = "/bin/sh";
  host.hostname  = com.example.service0;
  interface      = em0;
  ip4.addr       = 127.0.0.2;
  mount.fstab    = /usr/jail/com.example.service0.fstab;
  path           = /usr/jail/com.example.service0;
}

The /usr/jail/com.example.service0.fstab contains a single line:

/usr/jail/base /usr/jail/com.example.service0/base nullfs ro,noauto 0 0

I start the jail:

host# jail -v -f com.example.service0.conf -c com_example_service0
com_example_service0: run command: /sbin/ifconfig em0 inet 127.0.0.2 netmask 255.255.255.255 alias
com_example_service0: run command: /sbin/mount -t nullfs -o ro,noauto /usr/jail/base /usr/jail/com.example.service0/base
com_example_service0: jail_set(JAIL_CREATE) persist name=com_example_service0 host.hostname=com.example.service0 ip4.addr=127.0.0.2 path=/usr/jail/com.example.service0
com_example_service0: created
com_example_service0: run command in jail: /bin/sh
# exit
com_example_service0: jail_set(JAIL_UPDATE) jid=13 nopersist
host#

The jail is gone:

host# jls
   JID  IP Address      Hostname                      Path

However:

host# df -h | grep service0
# df -h                
Filesystem                         Size    Used   Avail Capacity  Mounted on
zroot/ROOT/default                  13G    475M     13G     4%    /
devfs                              1.0K    1.0K      0B   100%    /dev
zroot/tmp                           13G    108K     13G     0%    /tmp
zroot/usr/home                      13G    136K     13G     0%    /usr/home
zroot/usr/ports                     13G     96K     13G     0%    /usr/ports
zroot/usr/src                       13G     96K     13G     0%    /usr/src
zroot/var/audit                     13G     96K     13G     0%    /var/audit
zroot/var/crash                     13G     96K     13G     0%    /var/crash
zroot/var/log                       13G    148K     13G     0%    /var/log
zroot/var/mail                      13G     96K     13G     0%    /var/mail
zroot/var/tmp                       13G     96K     13G     0%    /var/tmp
zroot                               13G     96K     13G     0%    /zroot
zroot/jail                          13G    249M     13G     2%    /usr/jail
zroot/jail/com.example.service0     13G    4.0M     13G     0%    /usr/jail/com.example.service0
/usr/jail/base                      13G    249M     13G     2%    /usr/jail/com.example.service0/base

.. the /usr/jail/com.example.service0/base directory was not unmounted.

What do I need to do to get the directory to be correctly unmounted
when the jail ceases to exist?

M
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