This week's book giveaway is in the General Computing forum. We're giving away four copies of Arduino in Action and have Martin Evans, Joshua Noble, and Jordan Hochenbaum on-line! See this thread for details.
The exception shown has nothing to do with the "input" attribute. The "input" attribute defines the page that has the form on it--if validation fails this would be the page you'd want to return to. I don't recall if it's mandatory or not, but it's handy.
You don't describe the circumstances under which you get the exception, so it's hard to help.