Commit mirror fails with "no superclass method ..."

Summary

Pull mirroring fails with an inscrutable error message.

Steps to reproduce

Currently I haven't yet minimized the issue well enough to know what setup is necessary to trigger the failure. However, I am doing the following:

1. On https://gitlab.staging.haskell.org/ create a new project
2. Configure that project to mirror from git://git.haskell.org/ghc

Example Project

I cannot reproduce the issue on gitlab.com.

What is the current bug behavior?

Mirroring will fail with,

super: no superclass method `commit_email'; for #<User id:2 @root>

What is the expected correct behavior?

The repository should be mirrored.

Relevant logs and/or screenshots

I so far have been unable to find any sign of this failure in any of the GitLab logs. Any hints for how to get a backtrace from this would be greatly appreciated.

Output of checks

Results of GitLab environment info

System information
System:		
Proxy:		no
Current User:	root
Using RVM:	no
Ruby Version:	unknown
Gem Version:	
Bundler Version:1.17.1
Rake Version:	12.3.1
Redis Version:	unknown
Git Version:	2.19.2
Sidekiq Version:5.2.1
Go Version:	unknown

GitLab information
Version:	11.5.1-ee
Revision:	Unknown
Directory:	/nix/store/akm4c6d4jhcggpmcsq0g0kyaj6jxdp5b-gitlab-ee-11.5.1/share/gitlab
DB Adapter:	postgresql
DB Version:	10.6
URL:		https://gitlab.staging.haskell.org
HTTP Clone URL:	https://gitlab.staging.haskell.org/some-group/some-project.git
SSH Clone URL:	gitlab@gitlab.staging.haskell.org:some-group/some-project.git
Elasticsearch:	no
Geo:		no
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers: github

GitLab Shell
Version:	8.3.3
Repository storage paths:
- default: 	/var/gitlab/state/repositories
Hooks:		/var/gitlab/state/shell/hooks
Git:		git

Results of GitLab application Check

This is running under NixOS; sadly the application check does not currently pass although for somewhat silly reasons.

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)