Feed by mail $last_run always equals to -1 (at least in preview)

In my theme file, I’m checking the last_run and if it equals to -1 I display a selection of posts, else I select the post published since the last run, but in the preview when I display last_run it always equals to -1 (even if I already have some runs). Maybe I missed something.

Here is the code

<?php
global $newsletter; // Newsletter object
global $post; // Current post managed by WordPress
echo $last_run;
....
?>

Thanks for support.
Nicolas