adding custom fields to next and previous post link

Im trying to make my next and previous post buttons display my custom field but for some reason it doesnt display the custom field its just blank. Heres my code.

<div id="next-prev-links">
      <?php

      $previous_post = previous_post_link('%link', ''.$image_prev.'', TRUE);
      $next_post = next_post_link('%link',''.$image_next.'', TRUE);
      $image_prev = get_post_meta( $previous_post->ID, 'image', true);
      $image_next = get_post_meta( $next_post->ID, 'image', true);
      ?>

      <?php if ( $image_prev != '' ) : ?>
        <img src="https://wordpress.stackexchange.com/questions/9044/<?php echo $image_prev; ?>" alt="" />
      <?php endif; ?>

      <?php if ( $image_next != '' ) : ?>
        <img src="<?php echo $image_next; ?>" alt="" />
      <?php endif; ?>

      <?php previous_post_link('%link', 'Previous post', TRUE); ?>
      <?php next_post_link('%link', 'Next post', TRUE); ?>
</div>

Here’s the code for what you’re trying to do (accessing post custom value):

<?php    
global $post;

    $prev_post = get_adjacent_post();
    $next_post = get_adjacent_post( false, '', false );
    $prev_post_id = $prev_post->ID;
    $next_post_id = $next_post->ID;
    // this should, according to your code above, be the http://example.com/your/img.jpg string
    $prev_img_path = get_post_custom_values( 'your_key_name', $prev_post_id );
    $next_img_path = get_post_custom_values( 'your_key_name', $next_post_id );
    $prev_def_path = get_bloginfo('stylesheet_directory').'/default_next_img_inside_theme_folder.jpg';
    $next_def_path = get_bloginfo('stylesheet_directory').'/default_prev_img_inside_theme_folder.jpg';

    echo '<img src="'.!empty($prev_img_path) ? $prev_img_path : $prev_def_path.'" alt="Permalink to previous post" />';
    echo '<img src="'.!empty($next_def_path) ? $next_def_path : $next_img_path.'" alt="Permalink to next post" />';
?>