Rails slim form_for tag

I am having trouble getting the form to work after switching from regular ERB to slim files. Here is the shape I'm trying to render with a thin render:

= form_for @student, :url => students_path(@student), method: :post do |f|
    = f.hidden_field :student_id, :value => current_user.id
    = f.hidden_field :course_id, :value => group.id
    = submit_tag "Join this Class!", :class => "btn btn-primary pull-right join-button"

      

        
        
        
      

    

Here is the working code in the ERB file

<%= form_for @student, :url => students_path(@student), method: :post do |f| %>
    <%= f.hidden_field :student_id, :value => current_user.id %>
    <%= f.hidden_field :course_id, :value => group.id %>
    <%= submit_tag "+ Join", :class => "btn btn-primary pull-right join-button" %>
<% end %>

      

        
        
        
      

    

This is the error I am currently getting:

undefined local variable or method `f'