Scriptaculous Sortables - this.observer is not a function

Hit errors with the error message "this.observer is not a function" when releasing a dragged object using the excellent Sortables function from scriptaculous.
Much grief before I eventually found the problem: where all the other options settings require the value to be in quotes, the "onUpdate" parameter requires the function name NOT to be quoted
.

So - use onUpdate:showList not onUpdate:'showList'