gptzfsboot: error 128 during upgrade

[NASbox]

I just ran an upgrade from FreeNAS 11.2-U8 to FreeNAS 11.3-U4.1 using the GUI.
The reboot took a very long time, and I got a series of error messages
(I might have missed one or two):

gptzfsboot: error 128 LBA 32
gptzfsboot: error 128 LBA 544
gptzfsboot: error 128 LBA 7814036000
gptzfsboot: error 128 LBA 7814036512
gptzfsboot: error 128 LBA 7814037160
gptzfsboot: error 128 LBA 1
gptzfsboot: error 128 LBA 32
gptzfsboot: error 128 LBA 544
gptzfsboot: error 128 LBA 7814036000

and the system finally booted. The system appears to be running normally,
and I did a scrub of my boot pool which returned no errors.

Does anyone know what this is referring to? (Is there a log I should be lookling at?)
Is it likely a problem, or can I safely ignore it?

Any assistance is much appreciated.