do-release-upgrade is crashing when going from 16.04 to 18.04

[Robert Heller]

I recently upgraded a laptop from 14.04 to 16.04 using do-release-upgrade.  I 
has been running just fine with  16.04, but I just tried to update it to 18.04 
by running do-release-upgrade again, but do-release-upgrade is crashing.

The only *possible* weirdness is that I am using a local mirror (built with 
debmirror).  I did use this mirror to install a 18.04 VM (and have been 
successfully keeping the VM up-to-date).  And I did use the mirror to upgrade 
this laptop, another laptop, and (so far) one desktop machine from 14.04 to 
16.04.

What should I be looking for to debug this problem?


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