Rails undefined method `delete'

'delete'と書いていないのにエラー：

undefined method `delete' for true:TrueClass

もとのコード：

=form_for :school, url: schools_path do |f|

=check_box :kindergarten, { :multiple => true }, true, false

直したコード：

=form_for :school, url: schools_path do |f|

=f.check_box :kindergarten, { :multiple => true }, true, false