upgrade to squeeze fails for NFS-rooted system

[debian@good-with-numbers.com]

Upgrading lenny to squeeze fails with the following message during
"apt-get upgrade":

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Preparing to replace mount 2.13.1.1-1 (using .../mount_2.17.2-3.3_amd64.deb) ...
You have NFS mount points currently mounted, and this version of mount
requires that nfs-common be upgraded before NFS mounts will work.

Aborting install.
dpkg: error processing /var/cache/apt/archives/mount_2.17.2-3.3_amd64.deb (--unpack):
 subprocess pre-installation script returned error exit status 1
Errors were encountered while processing:
 /var/cache/apt/archives/mount_2.17.2-3.3_amd64.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)
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Yes, I do have an NFS mount--my whole filesystem:

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# mount
/dev/nfs on / type nfs (rw)
[...]
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So how do I upgrade?