Drawing rectangle on python Tkinter canvas that covers the entire canvas does not show border on top and left

I'm trying to create a rectangle inside a Tkinter (Python 2.7) canvas that is the same dimension as the canvas. Here is the relevant part of the code:

self.canvas = Canvas(self, width=100, height=100, backround="yellow")
self.canvas.create_rectangle(0,0,100,100)

This draws a rectangle but I can't see the left and top border of the rectangle. If I start the rectangle from let's say 5,5 instead of 0,0 I can see the border of the rectangle. Any ideas as to why this happens, and how I can get around it?