Problem installation validator

Hi, first of all, sorry for my english

I have installed a local validator on my notebook and seems to work 
correctly... It check all the error of local page

The problem was when the page is done and the validator return this error

/*Validation Output: 2 Errors

  (Validator local file "catalog") Line 1: Document is empty

  (Validator local file "catalog") Line 1: Start tag expected, '<' not 
found*/

Why this?

Thanks!

Onofrio