What am I doing wrong in this simple tkinter example?

[Yves S. Garret]

Hi all, I'm well into "Python Programming for the Absolute Beginner" in order to 
become more acquainted with the language.  However, when I got to page 304 and 
did my first example:

http://bin.cakephp.org/view/1107093008

And this is the error that I'm getting:

http://bin.cakephp.org/view/399711843

This is the code that came with the book:

http://bin.cakephp.org/view/514822432


What I don't understand is, why am I getting that error?  I've done diff -w
and the code looks for the most part the same.  What am I doing wrong when I
put the application together?