BUG #15446: Crash on ALTER TABLE

The following bug has been logged on the website:

Bug reference:      15446
Logged by:          Dmitry Molotkov
Email address:      aldarund@gmail.com
PostgreSQL version: 11.0
Operating system:   docker
Description:

postgres via docker.
latest tag dont work. 

2018-10-19 23:17:31.272 UTC [1] LOG:  listening on IPv4 address "0.0.0.0",
port 5432
2018-10-19 23:17:31.272 UTC [1] LOG:  listening on IPv6 address "::", port
5432
2018-10-19 23:17:31.280 UTC [1] LOG:  listening on Unix socket
"/var/run/postgresql/.s.PGSQL.5432"
2018-10-19 23:17:31.299 UTC [62] LOG:  database system was shut down at
2018-10-19 23:17:31 UTC
2018-10-19 23:17:31.306 UTC [1] LOG:  database system is ready to accept
connections
2018-10-19 23:17:53.903 UTC [1] LOG:  server process (PID 69) was terminated
by signal 11: Segmentation fault
2018-10-19 23:17:53.903 UTC [1] DETAIL:  Failed process was running: ALTER
TABLE "admin_interface_theme" ALTER COLUMN
"related_modal_background_opacity" TYPE varchar(5) USING
"related_modal_background_opacity"::varchar(5)
2018-10-19 23:17:53.903 UTC [1] LOG:  terminating any other active server
processes
2018-10-19 23:17:53.905 UTC [66] WARNING:  terminating connection because of
crash of another server process
2018-10-19 23:17:53.905 UTC [66] DETAIL:  The postmaster has commanded this
server process to roll back the current transaction and exit, because
another server process exited abnormally and possibly corrupted shared
memory.
2018-10-19 23:17:53.905 UTC [66] HINT:  In a moment you should be able to
reconnect to the database and repeat your command.
2018-10-19 23:17:53.905 UTC [1] LOG:  all server processes terminated;
reinitializing
2018-10-19 23:17:53.931 UTC [70] LOG:  database system was interrupted; last
known up at 2018-10-19 23:17:31 UTC
2018-10-19 23:17:53.940 UTC [71] FATAL:  the database system is in recovery
mode
2018-10-19 23:17:54.139 UTC [70] LOG:  database system was not properly shut
down; automatic recovery in progress
2018-10-19 23:17:54.145 UTC [70] LOG:  redo starts at 0/1651600
2018-10-19 23:17:54.151 UTC [70] LOG:  invalid record length at 0/16C9FE8:
wanted 24, got 0
2018-10-19 23:17:54.151 UTC [70] LOG:  redo done at 0/16C9FC0
2018-10-19 23:17:54.151 UTC [70] LOG:  last completed transaction was at log
time 2018-10-19 23:17:53.888672+00
2018-10-19 23:17:54.228 UTC [1] LOG:  database system is ready to accept
connections

Happens every time with 100% reproducibility. 
Work fine on postgres:10.5 docker tag.
It is just migration from
https://github.com/fabiocaccamo/django-admin-interface package.