diff --git a/src/drawer.js b/src/drawer.js
index 7567e354..f4ed36a6 100644
--- a/src/drawer.js
+++ b/src/drawer.js
@@ -384,7 +384,7 @@ $.Drawer.prototype = /** @lends OpenSeadragon.Drawer.prototype */{
 
         this.context.save();
         this.context.globalAlpha = opacity;
-        if (compositeOperation !== undefined && compositeOperation !== null) {
+        if (compositeOperation) {
             this.context.globalCompositeOperation = compositeOperation;
         }
         this.context.drawImage(
diff --git a/src/tiledimage.js b/src/tiledimage.js
index f3400ab3..bb432dd9 100644
--- a/src/tiledimage.js
+++ b/src/tiledimage.js
@@ -1330,7 +1330,7 @@ function drawTiles( tiledImage, lastDrawn ) {
         return;
     }
     var useSketch = tiledImage.opacity < 1 ||
-          (tiledImage.compositeOperation !== null && tiledImage.compositeOperation !== 'source-over');
+          (tiledImage.compositeOperation && tiledImage.compositeOperation !== 'source-over');
 
     var sketchScale;
     var sketchTranslate;