Debian testing DVD Installation Fails Into VirtualBox?

Hi,
the installation of current Debian Testing DVD into VirtualBox
fails when trying to install GRUB: the error was stating
"grub installation failed.Without the grub boot loader,the installed system will not boot"
what happen?because as in the manual ,
I used default settings during the graphical install.
(host is Ubuntu 11.04 64Bit and guest is Debian Testing 64Bit DVD1)
Anyone have any idea on how to fix this?
Thanks!
