ActionView::TemplateError (You have a nil object when you di

actually i am trying to add some values into a dropdown listfrom an
xml file.while running my application i am getting one error
mentioned like this

ActionView::TemplateError (You have a nil object when you didn’t expect
it!
You might have expected an instance of Array.
The error occurred while evaluating nil.map) on line #23 of
app/views/campaigns/_top_section.html.haml:
20: %tr
21: %td{ :valign => :top }
22: .label #{t :campaign_type}:
23: = f.select :campaign_type, Setting.unroll(:campaign_type), {
:selected => (@campaign.campaign_type || “email_campaign”).to_sym }, {
:style => “width:300px” }

please help me