How To Dynamically Create HTML 5 Canvas

Here's a snippet:
canvases[i] = document.createElement('canvas');
canvases[i].style.width = '3px';
canvases[i].style.height = '100%';
canvases[i].getContext("2D");
canvases[i].fillRect(1,0,1,table_height);

I'm trying to dynamically create a canvas element. Later in the script it gets attached to the DOM, and this works fine if the last two lines are missing. I end up with a canvas element that is 3px wide and 100% of the containing div tall. However, the last two lines make it crash, with the error report 'canvases[i].fillRect is not a function'.

If I change the last two lines to:
var drawing = canvases[i].getContext("2D");
drawing.fillRect(1,0,1,table_height);

It says 'drawing is null'. Something seems to be going wrong with the getContext bit, because setting the height and width work fine. It behaves the same if I append the canvas element to the DOM before trying to do the drawing as well. This is in FF4.0.1 with a xhtml1-strict.dtd doctype and valid html.