Admin ajax request return 0 with die()

So i’m using this configuration for an AJAX call in the administrative area ( like this http://codex.wordpress.org/AJAX_in_Plugins#Ajax_on_the_Administration_Side )

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It’s just a test code, but it always return 0 instead of the string “attivato”. It also find the ajax url correctly, without a 404 error. What is the problem? 🙁