Failed to execute 'toDataURL' on 'HTMLCanvasElement': shadow canvases may not be exported

I have a canvas that was created by a user (me) by dragging and dropping an image into the dropzone. As part of saving the image, I call toDataURL.

var theCanvas = document.getElementById ("idCardCanvas");

var urlImage = theCanvas.toDataURL ();

This code worked for months when it was run from the same domain, littlecardeditor.com.

Now I am adding this feature to Radio3, http://radio3.io/ .

Both domains are Amazon S3 buckets, both have the same CORS configuration (I'm holding onto straws here, I don't understand why CORS is included as the images are coming from my hard drive and not from any domain).

I have dragged images from the desktop based on advice from another thread.

None of this matters. It works when I use it on littlecardeditor.com and fails with a "broken canvas" error when called on radio3.io.

Not sure what else to see. Any hints would be much appreciated! :-)