qemu-nbd: Failed to bind socket: No such file or directory

[白い熊＠相撲道]

I'm completely baffled...

Using qemu, I'm trying to mount a qcow2 image to manipulate its files. I 
insmoded the nbd kernel module. The block devices exist:

~$ ls -l /dev/nbd*
brw-rw---- 1 root disk 43,  0 Mar 24 18:09 /dev/nbd0
brw-rw---- 1 root disk 43,  1 Mar 24 18:09 /dev/nbd1
brw-rw---- 1 root disk 43, 10 Mar 24 18:09 /dev/nbd10
brw-rw---- 1 root disk 43, 11 Mar 24 18:09 /dev/nbd11
brw-rw---- 1 root disk 43, 12 Mar 24 18:09 /dev/nbd12
brw-rw---- 1 root disk 43, 13 Mar 24 18:09 /dev/nbd13
brw-rw---- 1 root disk 43, 14 Mar 24 18:09 /dev/nbd14
brw-rw---- 1 root disk 43, 15 Mar 24 18:09 /dev/nbd15
brw-rw---- 1 root disk 43,  2 Mar 24 18:09 /dev/nbd2
brw-rw---- 1 root disk 43,  3 Mar 24 18:09 /dev/nbd3
brw-rw---- 1 root disk 43,  4 Mar 24 18:09 /dev/nbd4
brw-rw---- 1 root disk 43,  5 Mar 24 18:09 /dev/nbd5
brw-rw---- 1 root disk 43,  6 Mar 24 18:09 /dev/nbd6
brw-rw---- 1 root disk 43,  7 Mar 24 18:09 /dev/nbd7
brw-rw---- 1 root disk 43,  8 Mar 24 18:09 /dev/nbd8
brw-rw---- 1 root disk 43,  9 Mar 24 18:09 /dev/nbd9

But:

~$ sudo qemu-nbd -c /dev/nbd0 disk.qcow2
Failed to bind socket: No such file or directory

What is going on? I've searched the web, but have no idea what the 
problem could be.

Does anyone have an idea what is the issue?
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白い熊＠相撲道