Chapter 7. Working with Nodes, Page 146

Location on the page: 
Code, 5th line from the bottom
Chapter: 
7. Working with Nodes
Error: 

function joke_form($node) {

Correction: 

Should be:
function joke_form(&$node) {

Description of the Error: 

In print, parameter $node should be passed by reference since it can be altered / augmented in within hook_form.