diff --git a/lib/pngstream.js b/lib/pngstream.js
index 22df95e..a897abc 100644
--- a/lib/pngstream.js
+++ b/lib/pngstream.js
@@ -41,6 +41,7 @@ var PNGStream = module.exports = function PNGStream(canvas, sync) {
       : 'streamPNG';
   this.sync = sync;
   this.canvas = canvas;
+  if ('streamPNG' == method) throw new Error('async png streaming not yet implemented');
   process.nextTick(function(){
     canvas[method](function(err, chunk, len){
       if (err) {