CRED Shortcode to grab title of post which the form resides in

I'm placing this in my themes function.php:

function post_name($atts, $content) {
global $post;
return $post->post_title;
}
add_shortcode('post_name','post_name');

and using this shortcode to call it:

[post_name]

But nothing is being returned in the Subject link of my CRED email form?

Any idea why this is not working? Please provide code samples I've been working on this all day with no luck