VFS: Can't find ext3 filesystem on dev loop0

I was trying to make kickstart iso for unattended installation of ESX 4.1. I followed the procedure mentioned here:

http://www.vmprofessional.com/index.php?content=kickstart_2

But this step

we need to extract the image, and mount it as a loopback device.

[root@esx01 ~]# gunzip initrd.img --suffix .img
[root@esx01 ~]# mount -o loop /root/initrd /mnt/floppy

fails asking for mentioning the filesystem. So I tried this:

mount -o loop -t ext2 initrd /mnt/floppy

It says:

VFS: Can't find ext2 filesystem on dev loop0

I tried this:

mount -o loop -t ext3 initrd /mnt/floppy

It says:

VFS: Can't find ext3 filesystem on dev loop0

I tried this:

mount -o loop -t vfat initrd /mnt/floppy

It says:

VFS: Can't find ext2 filesystem on dev loop0

FAT: Invalid media

Any idea how to make it work?

Thanks