"Your installation media couldn't be mounted"

I want to install Debian Hurd in a VM.
I downloaded debian-sid-hurd-amd64-NETINST-1.iso, and roughly followed 
the instructions at 
https://www.gnu.org/software/hurd/hurd/running/qemu.html, except: I 
created a 24G qcow2 disk image, gave the VM 8G of RAM, chose C instead 
of English as language in the installer:

qemu-img create -f qcow2 disk.qcow2 24G

qemu-system-x86_64 -m 8G -drive cache=writeback,file=disk.qcow2 -cdrom 
debian-sid-hurd-amd64-NETINST-1.iso -net nic,model=rtl8139 -net user -smp 1

But the installer then tells me "Your installation media couldn't be 
mounted. When installing from CD-ROM, this probably means that the disk 
was not in the drive. If so you can insert it and try again."

Immediately before, I briefly see a message "/hurd/iso9660fs: Could not 
find valid superblock".

Philipp