ZFS on Linux: Zpool import failed

We upgraded a Proxmox box today which was running ZFS and ran into this rather scary looking error:  
zpool: ../../lib/libzfs/libzfs_import.c:356: Assertion `nvlist_lookup_uint64(zhp->zpool_config, ZPOOL_CONFIG_POOL_GUID, &theguid) == 0′ failed.
  Zpools would not import and zpool status did not work. Resolved (so far, anyhow, still testing) by running:  
apt-get install zfsutils
  Another good reason to have test environments…