Empty string supplied as input when parsing content

I have this code:

<?php
function mb_find_my_image( $content ) {
    if( is_home() ) { /* if is home start */

      $dom = new domDocument;
      $dom->loadHTML($content);
      $dom->preserveWhiteSpace = false;


    } /* if is home end */
    return $content;
}
add_filter( 'the_content', 'mb_find_my_image' );
?>

However, it seems that I always get the following error:

Empty string supplied as input on line 6

What I’m I doing wrong here?