Can't mount thumb drive

[Steven Friedrich]

When I plug in my thumb drive (A Kingston Data Traveler 1G, FAT), dmesg
says:

ugen0.5: <Kingston DataTraveler 2.0> at usbus0
umass0 on uhub0
umass0: <Kingston DataTraveler 2.0, class 0/0, rev 2.00/1.00, addr 4> on
usbus0
umass0:  SCSI over Bulk-Only; quirks = 0xc100
umass0:7:0: Attached to scbus7
da0 at umass-sim0 bus 0 scbus7 target 0 lun 0
da0: <Kingston DataTraveler 2.0 1.04> Removable Direct Access SCSI device
da0: Serial Number 0681F150A2929B60
da0: 40.000MB/s transfers
da0: 980MB (2007040 512 byte sectors)
da0: quirks=0x3<NO_SYNC_CACHE,NO_6_BYTE>

but when I try to mount it using:
mount_msdosfs /dev/da0 /mnt

it squeals Invalid argument.

So I tried:
mount_msdosfs /dev/da0a /mnt
it squeals Invalid argument.

Both /dev/da0 and /dev/da0a exist.

What am I doing wrong?