Magento 2.2.5 to 2.2.6 upgrade error

We user varnish and redis cache on our server and had this same error. This is the procedure we followed for a successful upgrade.

composer require magento/product-community-edition 2.2.6 --no-update
composer update
rm -rf var/di/* var/generation/* var/cache/* var/log/* var/page_cache/*
redis-cli flushdb
curl -X 'PURGE' -H'X-Magento-Tags-Pattern: .*'  localhost:8072
php70 bin/magento setup:upgrade

I believe redis and/or varnish are the culprit in this issue.

I had the same issue with m2.2.6 and redis. Solving:

redis-cli flushall