initramfs issue with latest apt update

[timxjr1300]

I've been running my rock64 for some time and this morning I did a routine update which partially failed...

Hit:1 http://ports.ubuntu.com/ubuntu-ports bionic InRelease
Hit:2 http://ports.ubuntu.com/ubuntu-ports bionic-security InRelease                               
Hit:3 http://ppa.launchpad.net/ayufan/rock64-ppa/ubuntu bionic InRelease                           
Hit:4 http://ports.ubuntu.com/ubuntu-ports bionic-updates InRelease                                                              
Hit:5 http://ppa.launchpad.net/ayufan/all-ppa/ubuntu bionic InRelease                                                            
Hit:6 http://ppa.launchpad.net/certbot/certbot/ubuntu bionic InRelease                                         
Ign:7 http://download.webmin.com/download/repository sarge InRelease                                                           
Get:8 http://deb.ayufan.eu/orgs/ayufan-rock64/releases  InRelease [1343 B]                                                     
Hit:9 http://download.webmin.com/download/repository sarge Release                                           
Fetched 1343 B in 3s (492 B/s)                                                   
Reading package lists... Done
Building dependency tree       
/usr/sbin/mkinitramfs: 12: export: �USYBOXDIR: bad variable name
update-initramfs: failed for /boot/initrd.img-4.4.132-1075-rockchip-ayufan-ga83beded8524 with 2.
dpkg: error processing package initramfs-tools (--configure):
 installed initramfs-tools package post-installation script subprocess returned error exit status 2
Errors were encountered while processing:
 initramfs-tools
E: Sub-process /usr/bin/dpkg returned an error code (1)

Advice on how to fix this would be welcome