NoMethodError: line_code_for_position

Summary

NoMethodError: line_code_for_position occurs when I write some comments on a line of code.

Steps to reproduce

Write a comment on a line of code. This error occurs in some lines but not in all lines.

Expected behavior

The comment has to be saved without any error.

Actual behavior

Sometimes I got 500 errors.

Relevant logs and/or screenshots

Started POST "/xxx/xxx/notes" for 10.42.43.107 at 2017-02-09 05:30:14 +0000  Processing by Projects::NotesController#create as JSON
  Parameters: {"utf8"=>"?", "authenticity_token"=>"VoOHF4NKWMDJsi2+fKush8pLQo37P43K2dgi0rpSMDzRkRxuNC8N0KDyTcqrQKPg2ywD5LEL1SSAF4N5JVzEMQ==", "view"=>"inline", "line_type"=>"new", "merge_request_diff_head_sha"=>"33db6bdad3e0f1328020b6e27b2e0fbbbdf2e122", "target_type"=>"merge_request", "target_id"=>"4589", "note"=>{"commit_id"=>"", "line_code"=>"45cb6d37cb9d777cf3add56e007f920b2b2d6220_0_5", "noteable_id"=>"4589", "noteable_type"=>"MergeRequest", "type"=>"DiffNote", "position"=>"{\"old_path\":\"XXX/XXX.cs\",\"new_path\":\"XXX/XXX.cs\",\"old_line\":null,\"new_line\":5,\"base_sha\":\"e10ff00bb6a4dc0d82d14eaa2c8a2ff73d16cc44\",\"start_sha\":\"e10ff00bb6a4dc0d82d14eaa2c8a2ff73d16cc44\",\"head_sha\":\"33db6bdad3e0f1328020b6e27b2e0fbbbdf2e122\"}", "note"=>"Hello, world!"}, "commit"=>"Comment", "namespace_id"=>"xxx", "project_id"=>"xxx"}
Completed 500 Internal Server Error in 45ms (ActiveRecord: 4.9ms)

NoMethodError (undefined method `line_code_for_position' for nil:NilClass):
  lib/gitlab/diff/position.rb:136:in `line_code'
  app/models/diff_note.rb:152:in `set_line_code'
  app/services/notes/create_service.rb:37:in `execute'
  app/controllers/projects/notes_controller.rb:27:in `create'
  lib/gitlab/middleware/multipart.rb:93:in `call'
  lib/gitlab/request_profiler/middleware.rb:15:in `call'
  lib/gitlab/middleware/go.rb:16:in `call'
  lib/gitlab/middleware/readonly_geo.rb:29:in `call'

Output of checks

Results of GitLab application Check

Too long to paste but there's no problem.

Redis version >= 2.8.0? ... yes
Ruby version >= 2.1.0 ? ... yes (2.3.3)
Your git bin path is "/opt/gitlab/embedded/bin/git
Git version >= 2.7.3 ? ... yes (2.10.2)
Active users: 38

Checking GitLab ... Finished

Results of GitLab environment info

System information
System:         Ubuntu 14.04
Current User:   git
Using RVM:      no
Ruby Version:   2.3.3p222
Gem Version:    2.6.6
Bundler Version:1.13.7
Rake Version:   10.5.0
Sidekiq Version:4.2.7

GitLab information
Version:        8.16.4-ee
Revision:       93daa28
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     PostgreSQL
DB Version:     9.2.18
URL:            http://git.k.nexon.com
HTTP Clone URL: http://git.k.nexon.com/some-group/some-project.git
SSH Clone URL:  git@git.k.nexon.com:some-group/some-project.git
Elasticsearch:  no
Geo:            no
Using LDAP:     no
Using Omniauth: no

GitLab Shell
Version:        4.1.1
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 have no idea.