Heroku release fails on upgrading postgres plan

I need to upgrade heroku postgres from standard-0 to starndard-2, so I created the follower db...Check another protip for creating follower db.

This is the first time after I added release phase command, and release phase command is pretty simple like following

bundle exec rake db:migrate

When I promoted follower db...I received notification from heroku said "Release phase command failed to execute"
I thought this is because of new database, so I set old db back...

HOWEVER it was safe to fail release when detaching db!!!! I should have been more careful to see log.
Here is the message when you run heroku run pg:promote

Checking release phase...pg:promote succeeded. It is safe to ignore the failed detach DATABASE release