ruby - Rails error : param is missing or the value is empty: booking -

i'm building events app using rails. app offers both paid , free events. paid events i'm using stripe payment process. i'm hitting issue processing bookings free events. bookings saving none of paramaters applying themselves, hence i'm getting following error -

this simple_form free events. requires user indicate how many spaces wish reserve -

new.html.erb

            <%= simple_form_for [@event, @booking], id: "new_booking" |form| %>                 <% if @booking.errors.any? %>                     <h2><%= pluralize(@booking.errors.count, "error") %> prevented booking saving:</h2>               <ul>                     <% @booking.errors.full_messages.each |message| %>                   <li><%= message %></li>                 <% end %>               </ul>             <% end %>                <div class="form-group">                 <p>please confirm number of spaces wish reserve event.</p>                 <%= form.input :quantity, class: "form-control" %>             </div>                <p> free event. no payment required.</p>                 <div class="panel-footer">                     <%= form.submit :submit, label: 'confirm booking', class: "btn btn-primary" %>                   <% end %>               </div>      

my bookings_controller #show action has -

 def show     @event = event.find(params[:event_id])     @booking = @event.bookings.new(booking_params) end 

i'm trying handle logic both free , paid bookings in model,

def free_booking       self.valid?            if event.is_free?              save!           end   end       def paid_booking     # stripe payment logic end