Can't add "Assignee" list to issue board

Summary

Attempt to add "Assignee" list to issue board throws an error

https://www.dropbox.com/s/5bo3sh0yeqc8nn3/Screenshot%202019-02-14%2012.46.25.png?dl=0

Steps to reproduce

Go to issue board
Click "Add List"
Toggle to Assignee

Example Project

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What is the current bug behavior?

Attempt to add "Assignee" list to issue board throws an error: ""Something went wrong while fetching assignees list"

https://www.dropbox.com/s/5bo3sh0yeqc8nn3/Screenshot%202019-02-14%2012.46.25.png?dl=0

What is the expected correct behavior?

Create Assignee list

Relevant logs and/or screenshots

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Output of checks

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Results of GitLab environment info

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Results of GitLab application Check

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sudo gitlab-rake gitlab:check SANITIZE=true)
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sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
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Possible fixes

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