Are you doing this outside of Draw events? The first thing I can think of would be the canvas not yet containing the image because of code being called before anything finishes drawing.
Other (less likely) potential causes:
- Something being odd if you have application_surface disabled
The game should still draw to canvas though, so seems unlikely
- Something being odd if you have WebGL enabled
I know that Microsoft Edge has a bug where WebGL canvas returns blank images, but the demo worked fine for me in Chrome and Firefox with WebGL enabled, so it would have to be some system-specific quirk.