Block user API call on system / internal users results in 500 error

Summary

Trying to block internal internal users like GitLab Support Bot, GitLab Alert Bot and Ghost User via the API returns HTTP 500 Internal Server Error with response body

{"message":"Notification email can't be blank. Notification email is invalid"}

The call failing is correct behaviour, the response is not.

Steps to reproduce

On one of the internal users' id (GitLab Support Bot, GitLab Alert Bot or Ghost User) call

curl -v -X POST --header "PRIVATE-TOKEN: $GITLAB_TOKEN" "http://gitlab.local/api/v4/users/$USER_ID/block"

What is the current bug behavior?

API returns 500 Internal Server Error with message Notification email can't be blank. Notification email is invalid.

What is the expected correct behavior?

API returns 403 Forbidden

Relevant logs and/or screenshots

curl -v -X POST --header "PRIVATE-TOKEN: $GIT_TOKEN" "http://gitlab.local/api/v4/users/25/block"
*   Trying 192.168.56.101...
* TCP_NODELAY set
* Connected to gitlab.local (192.168.56.101) port 80 (#0)
> POST /api/v4/users/25/block HTTP/1.1
> Host: gitlab.local
> User-Agent: curl/7.54.0
> Accept: */*
> PRIVATE-TOKEN: xxx
>
< HTTP/1.1 500 Internal Server Error
< Server: nginx
< Date: Wed, 16 Sep 2020 15:26:53 GMT
< Content-Type: application/json
< Content-Length: 78
< Connection: keep-alive
< Cache-Control: no-cache
< Vary: Origin
< X-Content-Type-Options: nosniff
< X-Frame-Options: SAMEORIGIN
< X-Request-Id: 9jvH4z9rne6
< X-Runtime: 0.091856
<
* Connection #0 to host gitlab.local left intact
{"message":"Notification email can't be blank. Notification email is invalid"}%

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:		Ubuntu 18.04
Proxy:		no
Current User:	git
Using RVM:	no
Ruby Version:	2.6.6p146
Gem Version:	2.7.10
Bundler Version:1.17.3
Rake Version:	12.3.3
Redis Version:	5.0.9
Git Version:	2.27.0
Sidekiq Version:5.2.9
Go Version:	unknown

GitLab information
Version:	13.2.4-ee
Revision:	5f3e8be35c3
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	PostgreSQL
DB Version:	11.7
URL:		http://gitlab.local
HTTP Clone URL:	http://gitlab.local/some-group/some-project.git
SSH Clone URL:	git@gitlab.local:some-group/some-project.git
Elasticsearch:	no
Geo:		no
Using LDAP:	yes
Using Omniauth:	yes
Omniauth Providers: 

GitLab Shell
Version:	13.3.0
Repository storage paths:
- default: 	/var/opt/gitlab/git-data/repositories
GitLab Shell path:		/opt/gitlab/embedded/service/gitlab-shell
Git:		/opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check

Checking GitLab subtasks ...
Checking GitLab Shell ...
GitLab Shell: ... GitLab Shell version >= 13.3.0 ? ... OK (13.3.0)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Internal API available: OK
Redis available via internal API: OK
gitlab-shell self-check successful
Checking GitLab Shell ... Finished
Checking Gitaly ...
Gitaly: ... default ... OK
Checking Gitaly ... Finished
Checking Sidekiq ...
Sidekiq: ... Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Checking Incoming Email ...
Incoming Email: ... Reply by email is disabled in config/gitlab.yml
Checking Incoming Email ... Finished
Checking LDAP ...
LDAP: ... Server: ldapmain
LDAP authentication... Success
LDAP users with access to your GitLab server (only showing the first 100 results)
User output sanitized. Found 4 users of 100 limit.
Checking LDAP ... Finished
Checking GitLab App ...
Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... yes
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
2/1 ... yes
1/4 ... yes
26/5 ... yes
33/6 ... yes
34/7 ... yes
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68/187 ... yes
70/188 ... yes
74/189 ... yes
74/190 ... yes
2/191 ... yes
75/192 ... yes
75/193 ... yes
Redis version >= 4.0.0? ... yes
Ruby version >= 2.5.3 ? ... yes (2.6.6)
Git version >= 2.22.0 ? ... yes (2.27.0)
Git user has default SSH configuration? ... yes
Active users: ... 7
Is authorized keys file accessible? ... yes
GitLab configured to store new projects in hashed storage? ... yes
All projects are in hashed storage? ... yes
Elasticsearch version 6.x - 7.x? ... skipped (elasticsearch is disabled)
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished