matplotlib fill command on axes plot problem

[Rominsky]

I am trying to use the fill command to draw a box around an object in
an image.  I can get the box drawn, but there seems to be a side
effect.  The fill command is adding white lines to the top and
sometimes right side of my axes plots.  I am using small images,
124x200, and after the fill command the axes plot is changed to
140x200, with the top 16 lines being all white.  I am running it in
interactive mode from the python command line.  Here is code that
reproduces the problem.

from pylab import *
im_array = zeros((124,200))
ion()
figure()
imshow(im_array)
hold(True)
x=40
y=40
fill([x-5,x+5,x+5,x-5],[y+5,y+5,y-5,y-5],ec='r', fill = False)
hold(False)

Any thoughts on what I might be doing wrong?