pmbootstrap install for qemu: /dev/installp2 is apparently in use by the system

I'm trying to follow the instructions here to create an installation for QEMU. I'm on Ubuntu 18.04.

I installed pmbootstrap via pip3:

pip3 install --user pmbootstrap

I ran pmboostrap init, accepting defaults, and pmbootstrap install, again accepting defaults.

The latter keeps failing with output:

[21:16:32] *** (1/5) PREPARE NATIVE CHROOT ***
[21:16:32] *** (2/5) CREATE DEVICE ROOTFS ("qemu-amd64") ***
[21:16:33] (rootfs_qemu-amd64) install
[21:16:34] (rootfs_qemu-amd64) install
[21:16:34] (rootfs_qemu-amd64) write /etc/os-release
[21:16:35] (rootfs_qemu-amd64) install
[21:16:35] (rootfs_qemu-amd64) mkinitfs virt
[21:16:38]  *** SET LOGIN PASSWORD FOR: 'user' ***
New password: 
Retype new password: 
passwd: password updated successfully
[21:16:45] NOTE: No valid keymap specified for device
[21:16:46] *** (3/5) PREPARE INSTALL BLOCKDEVICE ***
[21:16:46] (native) create qemu-amd64.img (728M)
[21:16:46] (native) mount /dev/install (qemu-amd64.img)
[21:16:46] (native) partition /dev/install (boot: 128M, reserved: 0M, root: the rest)
[21:16:47] (native) format /dev/installp2
[21:16:47] NOTE: The failed command's output is above the ^^^ line in the log file: /home/jnewsome/.local/var/pmbootstrap/log.txt
[21:16:47] ERROR: Command failed: (native) % mkfs.ext4 -O ^metadata_csum -F -q -L pmOS_root -N 100000 /dev/installp2
[21:16:47] See also: <https://postmarketos.org/troubleshooting>
Run 'pmbootstrap log' for details.

And the relevant bit from the log (full log attached):

(004065) [21:16:47] % sudo mount --bind /dev/loop12 /home/jnewsome/.local/var/pmbootstrap/chroot_native/dev/installp2
(004065) [21:16:47] (native) format /dev/installp2
(004065) [21:16:47] (native) % mkfs.ext4 -O ^metadata_csum -F -q -L pmOS_root -N 100000 /dev/installp2
/dev/installp2 is apparently in use by the system; will not make a filesystem here!
(004065) [21:16:47] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Any ideas?