REST api - variables for pipeline creation cause errors

Summary

While trying to provide variables to POST /api/v4/projects/:id/pipeline, a 500 response is returned

Steps to reproduce

Follow the documentation at https://docs.gitlab.com/ee/api/pipelines.html#create-a-new-pipeline

They say to post some kind of structure with fat arrows - maybe that's ruby? I've tried that, i've tried valid JSON (colons instead of fat arrows), i've tried as a parameter to the URL, i've tried form posting, form posting with variables[X] = Y...nothing works.

What is the current bug behavior?

Can't create a pipeline through REST with variables

What is the expected correct behavior?

pipelines can be created through REST with variables, not just through the trigger mechanism

Relevant logs and/or screenshots

Started POST "/api/v4/projects/241/pipeline?variables=[FILTERED]=%3E%20%27BRANCH%27,%20%27value%27%20=%3E%20%27CORE/master%27%7D]&ref=master" for 127.0.0.1 at 2019-03-08 10:38:51 -0500

TypeError (no implicit conversion of Symbol into Integer):
  /opt/gitlab/embedded/lib/ruby/gems/2.5.0/gems/activerecord-5.0.7.1/lib/active_record/nested_attributes.rb:480:in `[]'
  /opt/gitlab/embedded/lib/ruby/gems/2.5.0/gems/activerecord-5.0.7.1/lib/active_record/nested_attributes.rb:480:in `block in assign_nested_attributes_for_collection_association'
  /opt/gitlab/embedded/lib/ruby/gems/2.5.0/gems/activerecord-5.0.7.1/lib/active_record/nested_attributes.rb:480:in `map'
  /opt/gitlab/embedded/lib/ruby/gems/2.5.0/gems/activerecord-5.0.7.1/lib/active_record/nested_attributes.rb:480:in `assign_nested_attributes_for_collection_association'
  /opt/gitlab/embedded/lib/ruby/gems/2.5.0/gems/activerecord-5.0.7.1/lib/active_record/nested_attributes.rb:361:in `variables_attributes='
  /opt/gitlab/embedded/lib/ruby/gems/2.5.0/gems/activemodel-5.0.7.1/lib/active_model/attribute_assignment.rb:46:in `public_send'
  /opt/gitlab/embedded/lib/ruby/gems/2.5.0/gems/activemodel-5.0.7.1/lib/active_model/attribute_assignment.rb:46:in `_assign_attribute'
  /opt/gitlab/embedded/lib/ruby/gems/2.5.0/gems/activemodel-5.0.7.1/lib/active_model/attribute_assignment.rb:40:in `block in _assign_attributes'
  /opt/gitlab/embedded/lib/ruby/gems/2.5.0/gems/activemodel-5.0.7.1/lib/active_model/attribute_assignment.rb:39:in `each'
  /opt/gitlab/embedded/lib/ruby/gems/2.5.0/gems/activemodel-5.0.7.1/lib/active_model/attribute_assignment.rb:39:in `_assign_attributes'
  /opt/gitlab/embedded/lib/ruby/gems/2.5.0/gems/activerecord-5.0.7.1/lib/active_record/attribute_assignment.rb:26:in `_assign_attributes'
  /opt/gitlab/embedded/lib/ruby/gems/2.5.0/gems/activemodel-5.0.7.1/lib/active_model/attribute_assignment.rb:33:in `assign_attributes'
  /opt/gitlab/embedded/lib/ruby/gems/2.5.0/gems/attr_encrypted-3.1.0/lib/attr_encrypted/adapters/active_record.rb:28:in `perform_attribute_assignment'
  /opt/gitlab/embedded/lib/ruby/gems/2.5.0/gems/attr_encrypted-3.1.0/lib/attr_encrypted/adapters/active_record.rb:36:in `assign_attributes'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/ci/pipeline/chain/build.rb:9:in `perform!'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/ci/pipeline/chain/sequence.rb:19:in `block in build!'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/ci/pipeline/chain/sequence.rb:16:in `each'
  /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/ci/pipeline/chain/sequence.rb:16:in `build!'
  /opt/gitlab/embedded/service/gitlab-rails/app/services/ci/create_pipeline_service.rb:44:in `execute'
  /opt/gitlab/embedded/service/gitlab-rails/lib/api/pipelines.rb:61:in `block (2 levels) in <class:Pipelines>'

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:
Current User:	git
Using RVM:	no
Ruby Version:	2.5.3p105
Gem Version:	2.7.6
Bundler Version:1.16.6
Rake Version:	12.3.2
Redis Version:	3.2.12
Git Version:	2.18.1
Sidekiq Version:5.2.5
Go Version:	unknown
GitLab information
Version:	11.8.0
Revision:	ce13864
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
URL:		http://git.codemettle.com
HTTP Clone URL:	http://git.codemettle.com/some-group/some-project.git
SSH Clone URL:	git@git.codemettle.com:some-group/some-project.git
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers: google_oauth2
GitLab Shell
Version:	8.4.4
Repository storage paths:
default: 	/var/opt/gitlab/git-data/repositories
Hooks:		/opt/gitlab/embedded/service/gitlab-shell/hooks
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 >= 8.4.4 ? ... OK (8.4.4)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Redis available via internal API: OK
Access to /var/opt/gitlab/.ssh/authorized_keys: 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: ... Checking Reply by email ...
IMAP server credentials are correct? ... yes
Init.d configured correctly? ... skipped
MailRoom running? ... skipped
Checking Reply by email ... Finished
Checking Incoming Email ... Finished
Checking LDAP ...
LDAP: ... LDAP is disabled in config/gitlab.yml
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? ... no
Try fixing it:
sudo chown -R git /var/opt/gitlab/gitlab-rails/uploads
sudo find /var/opt/gitlab/gitlab-rails/uploads -type f -exec chmod 0644 {} ;
sudo find /var/opt/gitlab/gitlab-rails/uploads -type d -not -path /var/opt/gitlab/gitlab-rails/uploads -exec chmod 0700 {} ;
For more information see:
doc/install/installation.md in section "GitLab"
Please fix the error above and rerun the checks.
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: ...
27/1 ... yes
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Redis version >= 2.8.0? ... yes
Ruby version >= 2.3.5 ? ... yes (2.5.3)
Git version >= 2.18.0 ? ... yes (2.18.1)
Git user has default SSH configuration? ... yes
Active users: ... 132
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished