This happens when the browser doesn't support WebGL, and the canvas fallback module (boost-canvas.js) hasn't been included OR if the fallback module was included after the boost module.

If a fallback is required, make sure to include boost-canvas.js, and that it's included before boost.js.

Please note that the fallback module is not intended as a fully-featured one. Rather, it's a minimal implementation of the WebGL counterpart.