Controller node details page says: argument of type 'NoneType' is not iterable

/var/log/maas/regiond.log says:
2016-03-15 09:17:08 [-] Error on request (24) controller.set_active: argument of type 'NoneType' is not iterable
        Traceback (most recent call last):
          File "/usr/lib/python3.5/threading.py", line 862, in run
            self._target(*self._args, **self._kwargs)
          File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 794, in worker
            return target()
          File "/usr/lib/python3/dist-packages/twisted/_threads/_threadworker.py", line 46, in work
            task()
          File "/usr/lib/python3/dist-packages/twisted/_threads/_team.py", line 190, in doWork
            task()
        --- <exception caught here> ---
          File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 246, in inContext
            result = inContext.theWork()
          File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 262, in <lambda>
            inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
          File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 118, in callWithContext
            return self.currentContext().callWithContext(ctx, func, *args, **kw)
          File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 81, in callWithContext
            return func(*args,**kw)
          File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 448, in call_within_transaction
            return func_outside_txn(*args, **kwargs)
          File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 275, in retrier
            return func(*args, **kwargs)
          File "/usr/lib/python3.5/contextlib.py", line 30, in inner
            return func(*args, **kwds)
          File "/usr/lib/python3/dist-packages/maasserver/websockets/base.py", line 408, in set_active
            obj_data = self.get(params)
          File "/usr/lib/python3/dist-packages/maasserver/websockets/base.py", line 338, in get
            return self.full_dehydrate(obj)
          File "/usr/lib/python3/dist-packages/maasserver/websockets/base.py", line 194, in full_dehydrate
            return self.dehydrate(obj, data, for_list=for_list)
          File "/usr/lib/python3/dist-packages/maasserver/websockets/handlers/controller.py", line 79, in dehydrate
            data = super().dehydrate(obj, data, for_list=for_list)
          File "/usr/lib/python3/dist-packages/maasserver/websockets/handlers/machine.py", line 167, in dehydrate
            obj, data, for_list=for_list)
          File "/usr/lib/python3/dist-packages/maasserver/websockets/handlers/node.py", line 170, in dehydrate
            data["storage_layout_issues"] = obj.storage_layout_issues()
          File "/usr/lib/python3/dist-packages/maasserver/models/node.py", line 1029, in storage_layout_issues
            if (not boot_mounted and "arm64" in self.architecture and
        builtins.TypeError: argument of type 'NoneType' is not iterable

And storage for the rack/region controller in question is:
-kohlrabi(root) 304 : df
Filesystem 1K-blocks Used Available Use% Mounted on
udev 10257724 0 10257724 0% /dev
tmpfs 2055244 9324 2045920 1% /run
/dev/mapper/KOHLRABI-root 206293688 45544892 150246652 24% /
tmpfs 10276204 4 10276200 1% /dev/shm
tmpfs 5120 0 5120 0% /run/lock
tmpfs 10276204 0 10276204 0% /sys/fs/cgroup
/dev/mapper/KOHLRABI-srv 1548053708 1036454940 437930904 71% /srv
/dev/md0 944568 277576 601792 32% /boot
cgmfs 100 0 100 0% /run/cgmanager/fs
tmpfs 100 0 100 0% /run/lxcfs/controllers
tmpfs 2055244 0 2055244 0% /run/user/2501
-kohlrabi(root) 305 : lvdisplay -C
  LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert
  root KOHLRABI -wi-ao---- 200.00g
  root-trusty KOHLRABI -wi-a----- 100.00g
  srv KOHLRABI -wi-ao---- 1.46t
  swap KOHLRABI -wi-ao---- 100.00g
-kohlrabi(root) 306 : pvdisplay -C
  PV VG Fmt Attr PSize PFree
  /dev/md2 KOHLRABI lvm2 a-- 2.73t 893.37g
-kohlrabi(root) 307 : vgdisplay -C
  VG #PV #LV #SN Attr VSize VFree
  KOHLRABI 1 4 0 wz--n- 2.73t 893.37g
-kohlrabi(root) 308 : cat /proc/mdstat /proc/partitions
Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10]
md2 : active raid1 sdb3[1] sda3[2]
      2929060672 blocks super 1.2 [2/2] [UU]

md0 : active raid1 sda2[2] sdb2[3]
      976320 blocks super 1.2 [2/2] [UU]

unused devices: <none>
major minor #blocks name

   1 0 65536 ram0
   1 1 65536 ram1
   1 2 65536 ram2
   1 3 65536 ram3
   1 4 65536 ram4
   1 5 65536 ram5
   1 6 65536 ram6
   1 7 65536 ram7
   1 8 65536 ram8
   1 9 65536 ram9
   1 10 65536 ram10
   1 11 65536 ram11
   1 12 65536 ram12
   1 13 65536 ram13
   1 14 65536 ram14
   1 15 65536 ram15
   8 0 3907018584 sda
   8 1 96256 sda1
   8 2 976896 sda2
   8 3 2929191936 sda3
   8 4 976751616 sda4
   8 16 2930266584 sdb
   8 17 96256 sdb1
   8 18 976896 sdb2
   8 19 2929191936 sdb3
   9 0 976320 md0
   9 2 2929060672 md2
   8 32 1953481728 sdc
   8 33 1953480704 sdc1
 252 0 209715200 dm-0
 252 1 1572864000 dm-1
 252 2 104857600 dm-2
 252 3 104857600 dm-3