FrozenError: can't modify frozen String

https://sentry.gitlab.net/gitlab/gitlabcom/issues/773529/

FrozenError: can't modify frozen String
  (eval):107:in `block (3 levels) in draw_route'
  action_dispatch/routing/redirection.rb:53:in `path'
    block.call params, request
  action_dispatch/routing/redirection.rb:25:in `serve'
    uri = URI.parse(path(req.path_parameters, req))
  action_dispatch/routing/redirection.rb:21:in `call'
    serve Request.new env
  action_dispatch/routing/mapper.rb:17:in `block in <class:Constraints>'
    CALL  = ->(app, req) { app.call req.env }
...
(84 additional frame(s) were not displayed)

FrozenError: can't modify frozen String