Unacceptable merge request without discussion

Summary

I couldn't accept a merge request due to unresolved discussions. But there's no any discussion.

Steps to reproduce

For my case,

1. Turn on "Only allow merge requests to be merged if all discussions are resolved" option.
2. Receive a merge request.
3. Press "Approve" and "Rebase" button on the merge request page.

Expected behavior

I can accept this merge request without any discussion.

Actual behavior

It could not be accepted. It says there're unresolved discussions but there's nothing.

Relevant logs and/or screenshots

Output of checks

Results of GitLab application Check

Too long to paste. But everything is okay.

Results of GitLab environment info

System information
System:         Ubuntu 14.04
Current User:   git
Using RVM:      no
Ruby Version:   2.3.1p112
Gem Version:    2.6.6
Bundler Version:1.13.6
Rake Version:   10.5.0
Sidekiq Version:4.2.1

GitLab information
Version:        8.14.0-ee
Revision:       e45e0d4
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     PostgreSQL
DB Version:     9.2.18
URL:            http://xxx
HTTP Clone URL: http://xxx/some-group/some-project.git
SSH Clone URL:  git@xxx:some-group/some-project.git
Elasticsearch:  no
Geo:            no
Using LDAP:     no
Using Omniauth: no

GitLab Shell
Version:        4.0.0
Repository storage paths:
- default:      /var/opt/gitlab/git-data/repositories
Hooks:          /opt/gitlab/embedded/service/gitlab-shell/hooks/
Git:            /opt/gitlab/embedded/bin/git

Possible fixes

I just turned off the option.