Fabricjs+Node.js: fabric's canvas toDataURL calling toBuffer() not a method

I'm using the following code to try to see if I can load a canvas from a json string then generate a dataURL png for it:

var fabric=require('fabric');
var canvas = new fabric.fabric.Canvas();
var jsonStr='{"objects":[],"background":"rgba(0, 0, 0,0)","backgroundImage":"http://entropy.tmok.com/~gauze/canvas/any.gif","backgroundImageOpacity":1,"backgroundImageStretch":true,"overlayImage":"http://entropy.tmok.com/~gauze/canvas/frame.png","overlayImageLeft":0,"overlayImageTop":0}';

canvas.loadFromJSON(jsonStr);
img=canvas.toDataURL('png');

it errors on the toDataURL() line with:

/root/node-v0.8.16-linux-x86/node_modules/canvas/lib/canvas.js:190 return prefix + this.toBuffer().toString('base64'); ^

which tells me 'this' (which is a Canvas according to console.log) doesn't have a .toBuffer() method. am I doing something wrong or is this a bug in fabric's node module?

thanks.