slugged gem getting ActiveModel::MassAssignmentSecurity::Error in Admin::MaterialsController#update all of the sudden

materials_controller.rb

  def update
    @material = Material.find_using_slug(params[:id])
    redirect_to admin_material_url if @material.nil?

    respond_to do |format|
      if @material.update_attributes(params[:material])
        
        unless params[:material][:pdf].nil?
          @material.pdf = params[:material][:pdf]
          @material.save! 
        end
                   
        format.html { redirect_to edit_admin_material_path(@material), notice: 'Material Updated' }
        format.json { head :no_content, status: :success }
      else
        format.html { redirect_to edit_admin_material_path(@material), alert: @material.errors.full_messages }
        format.json { render json: @material.errors, status: :unprocessable_entity }
      end
    end
  end