[InvientCharts] CanvasRenderingContext2D is undefined in IE6/7


My VAADIN web application uses InvientCharts 0.8.6.
When using IE6/IE7 to browser, I got javascript error as the attachment.
No matter answer “yes” or “no”, no web page shows up, forever.
Does anyone how to fix it except not using IE?
PS: Our company only use IE…