vboxvfs: Not a directory

Hi,

I'm trying to use vboxfs on # uname -a
FreeBSD sjakie 12.0-CURRENT FreeBSD 12.0-CURRENT #4 r329516M: Sun Feb 18  
12:37:36 CET 2018      
ronald_at_sjakie:/data/ronald/obj-freebsd-current/data/ronald/freebsd-current/amd64.amd64/sys/GENERIC-NODEBUG   
amd64

I run VirtualBox on Windows 10 using FreeBSD 12 as a guest.
virtualbox-ose-additions-5.2.8 is installed by pkg.
I configured C:\Temp as 'win' in VirtualBox.

Modules are loaded:
kldstat | grep vbox
  5    2 0xffffffff82e26000 2d9e0    vboxguest.ko
13    1 0xffffffff82e7e000 1f320    vboxvfs.ko

[root_at_sjakie ~]# mount_vboxvfs win /mnt/win
[root_at_sjakie ~]# ls /mnt/win/
ls: /mnt/win/: Not a directory

I tried different Windows directories and different users/rights on the  
/mnt/win directory.
Am I doing something wrong?
I can't find a lot documentation about vboxvfs.

Regards,
Ronald.