This problem is a great example of why use strict is such a lifesaver. But perhaps lhoward was overly subtle. He provided a corrected code fragment without calling attention to the correction.

Read his code again, carefully. If necessary, copy/paste it into a file and diff it against your source.

If you're already doing use strict and aren't getting any error messages, then there's a different type of programming lesson waiting for you as when you compare the code snippets.