Cannot unmount CIFS mount?

Question

I am using Ubuntu 10.04.3 LTS. I have a NAS using CIFS protocol. I have mounted a share. I am not using fstab at all. Running mount yields the following:

//10.0.1.38/FOO on /home/s3backup/S3Backup/mnt/FOO type cifs (ro,mand)

When I attempt to unmount it I get the following:

$ sudo umount mnt/FOO           
This utility only unmounts cifs filesystems.
This utility only unmounts cifs filesystems.

I also tried the -l option and it yielded the same results. Any thoughts?

Thanks.