Can't select a mirror user

https://sentry.gitlap.com/gitlab/gitlabcom/issues/19176/

NoMethodError: undefined method `where' for #<Array:0x007fd77def3d08>
  app/controllers/autocomplete_controller.rb:21:in `users'
    @users = @users.where.not(id: current_user.id)
  lib/gitlab/middleware/multipart.rb:93:in `call'
    return @app.call(env) if encoded_message.blank?
  lib/gitlab/request_profiler/middleware.rb:14:in `call'
    @app.call(env)
  lib/gitlab/middleware/rails_queue_duration.rb:20:in `call'
    @app.call(env)
  lib/gitlab/metrics/rack_middleware.rb:29:in `block in call'
    retval = trans.run { @app.call(env) }
...
(94 additional frame(s) were not displayed)

NoMethodError: undefined method `where' for #<Array:0x007fd77def3d08>