[Plugin: Contact Form 7] Send/submit operation never completes

I have the same problem when I ad this in functions.php

add_action("wpcf7_mail_sent", "wpcf7_do_insert_post");
function wpcf7_do_insert_post($wpcf7) {
    var_dump($wpcf7);
    die();
}

I use WP 3.5.1
If I put just

add_action("wpcf7_mail_sent", "wpcf7_do_insert_post");
function wpcf7_do_insert_post($wpcf7) {
    return ($wpcf7);
}

it’s ok but if I add something else (echo ‘toto’;) I have the problem