Rails Checkboxes as array

I'm trying to pass multiple checkboxes as array to a db table. So i want to add multiple topics to a project.

= form_for @project, url: admin_projects_path, :html => { :multipart => true } do |p|
  %p
    =p.label :name
    =p.text_field :name 
  %p
    = p.label :topic
  %p
    = p.label "Value1"
    = p.check_box(:topic, {:multiple => true}, "value1", nil)
    = p.label "Value2"
    = p.check_box(:topic, {:multiple => true}, "value2", nil)
    = p.label "Value3"
    = p.check_box(:topic, {:multiple => true}, "value3", nil)
  %p
    = p.submit

but here topic returns nil in the projects, even whenever i check multiple checkboxes (i added to the whitelist params in the controller, so that cant be the cause)

What am i doing wrong here?