Python image boot halts at "failed to create mp dir "": read-only file system"

Using same cmd as in tutorial:

rumprun kvm -i \
   -b images/python.iso,/python/lib/python3.5 \
   -b examples/main.iso,/python/lib/python3.5/site-packages \
   -e PYTHONHOME=/python \
   -- examples/python.bin -m main

Full stacktrace:

ubuntu@ciaran:~/rumprun-packages/python3$ rumprun qemu -i -g '-nographic -vga none' -b images/python.iso,/python/lib/python3.5 -b examples/main.iso,/python/lib/python3.5/site-packages    -e PYTHONHOME=/python    -- examples/python.bin -m main

!!!
!!! NOTE: rumprun is experimental. syntax may change in the future
!!!

warning: TCG doesn't support requested feature: CPUID.01H:ECX.vmx [bit 5]
rump kernel bare metal bootstrap

WARNING: Processor claims to not support invariant TSC.
x86_initclocks(): TSC frequency estimate is 2000969160 Hz
x86_initclocks(): Using TSC for timekeeping
Copyright (c) 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005,
    2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016
    The NetBSD Foundation, Inc.  All rights reserved.
Copyright (c) 1982, 1986, 1989, 1991, 1993
    The Regents of the University of California.  All rights reserved.

NetBSD 7.99.34 (RUMP-ROAST)
total memory = 26664 KB
timecounter: Timecounters tick every 10.000 msec
timecounter: Timecounter "clockinterrupt" frequency 100 Hz quality 0
cpu0 at thinair0: rump virtual cpu
root file system type: rumpfs
kern.module.path=/stand/amd64/7.99.34/modules
mainbus0 (root)
pci0 at mainbus0 bus 0
pci0: i/o space, memory space enabled, rd/line, rd/mult, wr/inv ok
vendor 8086 product 1237 (host bridge, revision 0x02) at pci0 dev 0 function 0 not configured
vendor 8086 product 7000 (ISA bridge) at pci0 dev 1 function 0 not configured
vendor 8086 product 7010 (IDE mass storage, interface 0x80) at pci0 dev 1 function 1 not configured
vendor 8086 product 7113 (miscellaneous bridge, revision 0x03) at pci0 dev 1 function 3 not configured
virtio0 at pci0 dev 2 function 0
virtio0: Virtio Block Device (rev. 0x00)
ld0 at virtio0: Features: 0x10000054<INDIRECT_DESC,BLK_SIZE,GEOMETRY,SEG_MAX>
virtio0: allocated 270336 byte for virtqueue 0 for I/O request, size 128
virtio0: using 262144 byte (16384 entries) indirect descriptors
ld0: 352 KB, 2 cyl, 16 head, 63 sec, 512 bytes/sect x 704 sectors
virtio0: interrupting at pausebreak
virtio1 at pci0 dev 3 function 0
virtio1: Virtio Block Device (rev. 0x00)
ld1 at virtio1: Features: 0x10000054<INDIRECT_DESC,BLK_SIZE,GEOMETRY,SEG_MAX>
virtio1: allocated 270336 byte for virtqueue 0 for I/O request, size 128
virtio1: using 262144 byte (16384 entries) indirect descriptors
ld1: 352 KB, 2 cyl, 16 head, 63 sec, 512 bytes/sect x 704 sectors
virtio1: interrupting at pausebreak
timecounter: Timecounter "bmktc" frequency 1000000000 Hz quality 100
vendor 8086 product 1237 (host bridge, revision 0x02) at pci0 dev 0 function 0 not configured
vendor 8086 product 7000 (ISA bridge) at pci0 dev 1 function 0 not configured
vendor 8086 product 7010 (IDE mass storage, interface 0x80) at pci0 dev 1 function 1 not configured
vendor 8086 product 7113 (miscellaneous bridge, revision 0x03) at pci0 dev 1 function 3 not configured
mounted tmpfs on /tmp
rumprun: failed to create mp dir "": Read-only file system

=== bootstrap failed
rump kernel halting...
syncing disks... done
unmounting file systems...
unmounted /dev/ld0a on /python/lib/python3.5 type cd9660
unmounted tmpfs on /tmp type tmpfs
unmounted rumpfs on / type rumpfs
unmounting done
halted

rumpkernel / rumprun-packages

Python image boot halts at "failed to create mp dir "": read-only file system" #143