the_content breaks Shadowbox JS?

I’m using the following shortcode function to pull content from another post but it breaks Shadowbox’s ability to apply the rel attribute to the appropriate links.

function digwp_includeContentShortcode( $atts ) {
          $thepostid = (int) $atts['postidparam'];

          if ( ! $post = get_post( $thepostid ) )
               return '<em>Invalid post/page ID.</em>';

          return apply_filters( 'the_content', $post->post_content );
}
add_shortcode("digwp_include", "digwp_includeContentShortcode");

I’m guessing apply_filters is overriding SB plugin functionality but I’m not sure. In this post, the plugin author, Matt Martz, suggests that the_content should work whereas get_the_content may not. Obviously this is not the case here. Suggestions?

http://ww.wp.xz.cn/extend/plugins/shadowbox-js/