Problem with umount and fsid?

I get an error when umounting a FAT filesystem on a USB flash drive.  It
appears the device is properly unmounted.  Is this a case that needs to be
fixed in our fsid code?  It happens every time I unmount this device.

laptop# mount
/dev/ad0s4a on / (ufs, local)
devfs on /dev (devfs, local)
/dev/ad0s4e on /usr (ufs, local, soft-updates)
/dev/ad0s4f on /home (ufs, local, soft-updates)
/dev/ad0s1 on /dos (msdosfs, local)
procfs on /proc (procfs, local)
/dev/da0s1 on /thumb (msdosfs, local)
/dev/ad0s5 on /linux (ext2fs, local)
laptop# camcontrol inquiry da0
pass1: <Trek ThumbDrive Smart 1.11> Removable Direct Access SCSI-2 device
pass1: Serial Number
pass1: 1.000MB/s transfers
laptop# umount /thumb
umount: unmount of /thumb failed: No such file or directory
umount: retrying using path instead of file system ID
laptop# mount | grep da0
laptop#

Thanks,
Nate