JSLint does not like the increment and decrement operators. Replace it with i += 1 or add the plusplus: true directive to the top of your file (if you're not sure how to set JSLint directives, here's an example. They are set in a normal comment at the top of your file):
/*jslint plusplus: true */
From the JSLint docs:
The ++ (increment) and -- (decrement) operators have been known to
contribute to bad code by encouraging excessive trickiness. They are
second only to faulty architecture in enabling to viruses and other
security menaces.
Completely ridiculous rule? You can make your own mind up...
