Processing triggers for shim-signed (1.17~14.04.1+0.8-0ubuntu2) ...
Use of uninitialized value $ret in string eq at /usr/share/perl5/Debconf/FrontEnd/Passthrough.pm line 212, <GEN0> line 22.
Use of uninitialized value $_[1] in join or string at /usr/share/perl5/Debconf/DbDriver/Stack.pm line 111, <GEN0> line 22.
Use of uninitialized value $_[1] in join or string at /usr/share/perl5/Debconf/DbDriver/Stack.pm line 111, <GEN0> line 22.
Use of uninitialized value $ret in string eq at /usr/share/perl5/Debconf/FrontEnd/Passthrough.pm line 212, <GEN0> line 29.
Use of uninitialized value $_[1] in join or string at /usr/share/perl5/Debconf/DbDriver/Stack.pm line 111, <GEN0> line 29.
[...]
This indicates a problem with the debconf prompting code. What program were you running to do the upgrade?
The output in your dpkg upgrade log shows:
Processing triggers for shim-signed (1.17~14. 04.1+0. 8-0ubuntu2) ... perl5/Debconf/ FrontEnd/ Passthrough. pm line 212, <GEN0> line 22. perl5/Debconf/ DbDriver/ Stack.pm line 111, <GEN0> line 22. perl5/Debconf/ DbDriver/ Stack.pm line 111, <GEN0> line 22. perl5/Debconf/ FrontEnd/ Passthrough. pm line 212, <GEN0> line 29. perl5/Debconf/ DbDriver/ Stack.pm line 111, <GEN0> line 29.
Use of uninitialized value $ret in string eq at /usr/share/
Use of uninitialized value $_[1] in join or string at /usr/share/
Use of uninitialized value $_[1] in join or string at /usr/share/
Use of uninitialized value $ret in string eq at /usr/share/
Use of uninitialized value $_[1] in join or string at /usr/share/
[...]
This indicates a problem with the debconf prompting code. What program were you running to do the upgrade?