alpine-2.6.6-x86 won't boot with VirtualBox

Host: Linux gentoo-desktop 3.10.7-gentoo-r1
VB Ver: 4.1.26

Booting the 2.6.6 iso in VirtualBox gives the following error:

"Booting kernel failed: Bad file number" - the message is repeated every second or so.

The sha1 checksum also passes.

I did find:

append initrd=/boot/grsec.gz alpine_dev=usbdisk:vfat modules=loop,squashfs,sd-mod,usb-storage quiet

In the syslinux.cfg but i'm not entirely sure if it makes a difference that VirtualBox is reading the iso as a cd-rom - should it be installed via USB?

Thanks.