sketch.js.diff

diff --git a/js/sketch.js b/js/sketch.js
index 1f5c62f..7048e55 100644
--- a/js/sketch.js
+++ b/js/sketch.js
@@ -96,7 +96,7 @@ var __slice = Array.prototype.slice;
       return this.redraw();
     };
     Sketch.prototype.onEvent = function(e) {
-      if (e.originalEvent && e.originalEvent.targetTouches) {
+      if (e.originalEvent && e.originalEvent.targetTouches && e.originalEvent.targetTouches[0]) {
         e.pageX = e.originalEvent.targetTouches[0].pageX;
         e.pageY = e.originalEvent.targetTouches[0].pageY;
       }