ZIS OAuth error

Hi, I'm trying a tutrial for ZIS here, https://developer.zendesk.com/documentation/integration-services/zis-tutorials/zendesk-app-admin-interface/app-oauth/ .

But it failed at Slack OAuth process in Part 2.
https://developer.zendesk.com/documentation/integration-services/zis-tutorials/zendesk-app-admin-interface/zis-obtain-oauth-token/#kick-off-a-slack-oauth-flow

I got an error after clicked the redirect url, which was retuened from /api/services/zis/connections/oauth/start/{integration_key}.
The redirected URL was https://zis.zendesk.com/api/services/zis/connections/oauth/callback?code=....&state=.... and the error body was

{"errors":[{"status":"400","code":"1301","detail":" : oauth2: server response missing access_token"}]}

This is different result from the tutorial. There was no verification_token parameter. Demo app has the same error.

What am I missing in the tutorial?