While prototyping a page recently, I decided to hold off on passing in parameters until I had a good sense of what their names should be. So, I had something similar to:
Clearly this is a broken construct and Tapestry generally recognized it. I was greeted with the following error message:
"Parameter expression was null or contained only whitespace."
And a highlighted block of my template. Unfortunately, the highlighted block is not where the problem was. Likewise, the error message really didn't indicate what parameter was broken of what component. After a little bit of guess-and-check, I caught the obvious error. It'd be nice if Tapestry handled this for me though.