error in combination with simpletest

when simpletest is on, and i turn on coder, i get this error :

Fatal error: require_once() [function.require]: Failed opening required 'Text/Diff.php' (include_path='.:/usr/lib/php/extensions:sites/all/modules/coder/scripts/coder_format/tests') in /Users/kimmekes/Sites/drupal6/sites/all/modules/coder/scripts/coder_format/tests/all.test