$content is an empty string?

I’m making a plugin to parse my post content with PHP, and I have this line of 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, I’m always getting a “Empty string supplied as input” error.
I’m guessing that $content variable is the problem, but I don’t know what’s wrong.

I’m testing this using WAMP on my local computer.

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