xfs_repair refuses to run on cleanly mountable partition

[Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx>]

x4 ~ # xfs_repair -V
xfs_repair version 3.2.0-alpha1

x4 ~ # mount -o logbsize=256k /dev/sdc1 /mnt
...
[ 6419.592649] XFS (sdc1): Mounting Filesystem
[ 6419.642480] XFS (sdc1): Ending clean mount

x4 ~ # xfs_info /dev/sdc1
meta-data=/dev/sdc1              isize=256    agcount=4, agsize=61047552 blks
         =                       sectsz=4096  attr=2, projid32bit=0
         =                       crc=0
data     =                       bsize=4096   blocks=244190208, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0
log      =internal               bsize=4096   blocks=119233, version=2
         =                       sectsz=4096  sunit=1 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0

x4 ~ # umount /mnt

x4 ~ # xfs_repair /dev/sdc1
Phase 1 - find and verify superblock...
Phase 2 - using internal log
        - zero log...
ERROR: The filesystem has valuable metadata changes in a log which needs to
be replayed.  Mount the filesystem to replay the log, and unmount it before
re-running xfs_repair.  If you are unable to mount the filesystem, then use
the -L option to destroy the log and attempt a repair.
Note that destroying the log may cause corruption -- please attempt a mount
of the filesystem before doing this.

-- 
Markus