function calcRoute() {

        var start = document.getElementById("start_").value;

        var end = document.getElementById("end_").value;

        var request = {

            origin: start,

            destination: end,

            travelMode: google.maps.TravelMode.DRIVING

        };

        directionsService.route(request, function (response, status) {

            if (status == google.maps.DirectionsStatus.OK) {

                directionsDisplay.setDirections(response);

            }

        });

    }

Gives an error message in Chrome:

Uncaught TypeError: Cannot call method 'setDirections' of undefined

Could anyone suggest fixing this?