Cannot convert string into integer error message

maps_controller.rb

def show
    @map = Map.find(params[:id])
    loc_one = @map.from_location_id.as_json(:only => [:latitude, :longitude])
    loc_two = @map.to_location_id.as_json(:only => [:latitude, :longitude])
    dave_loc = [ loc_one, loc_two ]
    dave_loc = dave_loc.map do |marker|
      { lng: marker['longitude'], lat: marker['latitude'] }
    end
    map = [ dave_loc ]
    @mapz = map.to_json
    
    respond_to do |format|
      format.html # show.html.erb
      format.json { render json: @map }
    end
  end