I'm having trouble with the error in the title.

It works fine in a local environment, and I don't know where the problem lies.
Checking heroku log, TypeError (no implicit conversion of nil into String)
Is displayed.
I had an article with a similar error, but I'm not sure what to look for because the situation is so different.

TypeError (no implicit conversion of nil into String):
What I tried

when building heroku

WARN --: ActsAsTaggableOn.delimiter is deprecated and will be removed from v4.0+, use a ActsAsTaggableOn.default_parser instead

I was wondering if this is the cause. (I don't know if this is the cause in the first place.)
I googled the alert and tried changing the gem version, but it didn't work.

Originally gem'acts-as-taggable-on'
Trial 1 → gem'acts-as-taggable-on','~>6.0'
Trial 2 → gem'acts-as-taggable-on', :git =>'https://github.com/F3pix/acts-as-taggable-on'

With the second method I tried, it seems that the migration file needs to be recreated.
Index name'index_taggings_on_tag_id' on table'taggings' already exists
I got an error.

I want to solve the first TypeError,
I don't know if this ActsAsTaggableOn.delimiter is the cause, so I returned to ActsAsTaggableOn.delimiter and asked a question on stackoverflow.

Supplemental information (FW/tool ​​version, etc.)

rails 5.1.4
ruby 2.5.8

  • Answer # 1

    When recreating the application on Heroku and deploying from 0 to heroku again, TypeError does not appear and it is displayed like the local environment.

    I haven't changed anything special, so I don't know how to figure out what went wrong, but it doesn't seem to matter about ActsAsTaggableOn.