I have written this innocent javascript code, which lets the user create two markers and plot the route between them. It doesnt work, instead, it gives a weird error:

Uncaught TypeError: Cannot read property 'ya' of undefined

Can someone suggest me whats wrong here:

// called upon a click
GEvent.addListener(map, "click", function(overlay,point) {
    if (isCreateHeadPoint) {
        // add the head marker
        headMarker = new GMarker(point,{icon:redIcon,title:'Head'});
        map.addOverlay(headMarker);
        isCreateHeadPoint = false;
    } else {
        // add the tail marker
        tailMarker = new GMarker(point,{icon:greenIcon,title:'Tail'});
        map.addOverlay(tailMarker);
        isCreateHeadPoint = true;
        // create a path from head to tail
        direction.load("from:" + headMarker.getPoint().lat()+ ", " + headMarker.getPoint().lng()+ " to:" + tailMarker.getPoint().lat() + "," + tailMarker.getPoint().lng(), { getPolyline: true, getSteps: true }); 
        // display the path
        map.addOverlay(direction.getPolyline());
    }
});