Filled stops working when the rectangle is inverted

[chrisws]

x1 = 20
x2 = 100
y1 = 20
y2 = 100
while 1
  print x1 + " " + y1 + " " + x2 + " " + y2
  rect x1, y1, x2, y2, 1 filled
  rect x1, y1, x2, y2, 2
  y1+=1
  delay 50
  cls
wend