no featured image with RSS feed

Hi Rikard,
The tutorial :
All you have to do is open your theme’s functions.php file and add the following code :

function rss_post_thumbnail($content) {
global $post;
if(has_post_thumbnail($post->ID)) {
$content = ‘<p>’ . get_the_post_thumbnail($post->ID) .
‘</p>’ . get_the_content();
}
return $content;
}
add_filter(‘the_excerpt_rss’, ‘rss_post_thumbnail’);
add_filter(‘the_content_feed’, ‘rss_post_thumbnail’);

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I do it, but nathing change.
There is no thumbnail ????

Thanks