belately thought of trying: running sudo update-initramfs -k all -c produced no errors for both currently installed kernels:
rachel@spitfire:~$ sudo update-initramfs -k all -c update-initramfs: Generating /boot/initrd.img-4.15.0-21-generic update-initramfs: Generating /boot/initrd.img-4.15.0-20-generic
FWIW...
belately thought of trying: running sudo update-initramfs -k all -c produced no errors for both currently installed kernels:
rachel@spitfire:~$ sudo update-initramfs -k all -c img-4.15. 0-21-generic img-4.15. 0-20-generic
update-initramfs: Generating /boot/initrd.
update-initramfs: Generating /boot/initrd.
FWIW...