StuckCiJobsWorker: Psych::SyntaxError: (<unknown>): did not find expected ',' or '}' while parsing a flow mapping at line 1 column 2

https://sentry.gitlab.net/gitlab/gitlabcom/issues/1024680/

Psych::SyntaxError: (<unknown>): did not find expected ',' or '}' while parsing a flow mapping at line 1 column 2
  psych.rb:456:in `parse'
    parser.parse yaml, filename
  psych.rb:456:in `parse_stream'
    parser.parse yaml, filename
  psych.rb:390:in `parse'
    parse_stream(yaml, filename: filename) do |node|
  psych.rb:349:in `safe_load'
    result = parse(yaml, filename: filename)
  gitlab/serializer/ci/variables.rb:15:in `load'
    object = YAML.safe_load(string, [Symbol])
...
(149 additional frame(s) were not displayed)

(<unknown>): did not find expected ',' or '}' while parsing a flow mapping at line 1 column 2