Cannot umount kerberized nfsv4 shares - always requires lazy umount

I have a ubuntu desktop with which I have a kerberized nfs4 mount from a server mounted at /home/brisbane . When I try to umount, I get the error

#> umount /home/brisbane
/home/brisbane was not found in /proc/mounts
/home/brisbane was not found in /proc/mounts

Indeed, the entry in /proc/mounts that should correspond to that mount has the suffix "\040(deleted)" over what I would expect

#######cat /proc/mounts#################
cplxfs3.physics.ox.ac.uk:/home/brisbane /home/brisbane\040(deleted) nfs4 rw,nosuid,nodev,noexec,relatime,vers=4,rsize=1048576,wsize=1048576,namlen=255,hard,proto=tcp,port=0,timeo=600,retrans=2,sec=krb5i,clientaddr=163.1.246.45,minorversion=0,local_lock=none,addr=163.1.74.51 0 0
####################################

#>umount -fl /home/brisbane
Does not give any errors, and allows the /home/brisbane line to dissapear from /proc/mounts

This problem occurs for all kerberized mounts.
If I shutdown the desktop machine, it hangs unless I manually unmount in this way.

 Is it safe to always umount in this way? I intend to add nfs4 mounted home directories to a number of desktop clients and this seems like a bit of a kludge.

Has anyone managed to set up kerberized nfs4 without the need to lazy unmount the filesystems?

I am running a ubuntu_precise client.