First argument in form cannot be nil error

company.html.erb

<div class="content pad10">
	<%= form_for @department do |f| %>
	<p>
		<%= f.label :department, 'Department Name' %>
		<%= f.text_field 'department' %>
	</p>
	<p>
		<%= f.label :parent %>
		<% department_array = @departments.all.map { |department| [department.name, department.id + 1] } %>
		<%= f.select :parent, options_for_select(department_array) %>
	</p>
	<%= f.submit 'Add Department', class: 'btn-std' %>
	<% end %>
</div>

dashboard_controller.rb

class DashboardController < ApplicationController
  before_action :require_login
  before_filter :init

  def index
    @departments.each do |department|
      childfetch = Department.where(parent: "#{department.name}").select('name')
      instance_variable_set("@#{department.name}Children".gsub(' ','_'), childfetch.map { |child| "#{child.name}" }.join(',') )
    end
  end
  def office

  end
  def preferences

  end
  def company
  end

  private

  def require_login
    unless session[:user_id].present?
      redirect_to root_url
    end
  end
  def init
    @current_user = session[:user_id]
    @departments = Department.where(parent: '')
  end
end

department_controller.rb

class DepartmentController < ApplicationController
  def index
  end
end