against the error

I, [2017-12-01T18:48:32.443015 #5]  INFO -- :   Rendered events/_form.html.haml (1.3ms)
I, [2017-12-01T18:48:32.443245 #5]  INFO -- :   Rendered events/new.html.haml within layouts/application (4.9ms)
I, [2017-12-01T18:48:32.443541 #5]  INFO -- : Completed 500 Internal Server Error in 50ms (ActiveRecord: 2.0ms)
F, [2017-12-01T18:48:32.445841 #5] FATAL -- :
ActionView::Template::Error (Illegal element: classes and ids must have values.):
     8:       = ticket.input :remote_ticket_id
     9:     = f.input :event_type, :collection => Event::TYPES
    10:     = f.association :track, collection: @conference.tracks
    11: # Removed for Malmö Conference
    12: #    = f.association :subcategory, collection: @conference.subcategories
    13:     = f.input :time_slots, :as => :select, :collection => timeslots
    14:     = f.input :public, as: :inline_boolean, :hint => "Is this event visible in the public schedule?"
  app/views/events/_form.html.haml:11
  app/views/events/new.html.haml:5:in `block (3 levels) in _app_views_events_new_html_haml__4385781608837283215_70231887247240'
  app/helpers/web_app_theme_helper.rb:11:in `inner'
  app/views/events/new.html.haml:4:in `block (2 levels) in _app_views_events_new_html_haml__4385781608837283215_70231887247240'
  app/helpers/web_app_theme_helper.rb:7:in `content'
  app/views/events/new.html.haml:2:in `block in _app_views_events_new_html_haml__4385781608837283215_70231887247240'
  app/helpers/web_app_theme_helper.rb:3:in `block'
  app/views/events/new.html.haml:1:in `_app_views_events_new_html_haml__4385781608837283215_70231887247240'
  app/controllers/events_controller.rb:211:in `new'