I can't mount yaffs2ram file system on linux-2.6.12-mm2

I make device like below.

mknod /dev/mtd0 c 90 0
mknod /dev/mtd1 c 90 2

but whent I try to mount a yaffs2ram file system. I get an error:

[root@localhost 2.6.12]# mount -v -t yaffs2ram /dev/mtd0 /mnt/nand
mount: /dev/mtd0 is not a block device

So I make mtd0 to block device. like this:

[root@localhost 2.6.12]# mknod /dev/mtd0 b 90 0

and I try to mount again. But I get an error, too:

[root@localhost 2.6.12]# mount -v -t yaffs2ram /dev/mtd0 /mnt/nand
mount: /dev/mtd0 is not a valid block device

How can I mount yaffs2ram file system???

I'm off to debug this, but any pointers as to what to look for would be greatly appreciated.

Tae-hoon Lee