I have an Ubuntu 12.04 guest running on an Ubuntu 13.10 host. I created a shared folder in the host following the documentation. When I try to mount the folder in the guest the result is:
$ sudo mount -t vboxsf -o uid=$UID,gid=$GID shared ~/host
gid= requires an argument (i.e. gid==<arg>)
I found no results when searching online for this error.