Page Chapter 10. The Form API, Page 163

Location on the page: 
Passing Data Along with form_set_value() section.
Error: 

Omission of the $form argument in the hook_validate() sig if you are using form_set_value().

Correction: 

Show the needed $form argument in the _validate() signature:
_validate($form_id, $form_values, $form) {
...

Description of the Error: 

It would be helpful to show how the $form variable became available in the form_set_value() statement.