I have done a test in IE 8 with a simple page and two select lists on it. When you get this error does it specify which part of the code this is coming from? I do not get this error when I use the tool in IE8. If the update works for you that seems ok but I'm curious why it happens for you in the first place. Do you have a link to the live page?
When you replaced the function call did you do this for all instances of the call or just a single instance?