diff --git a/mailsystem.install b/mailsystem.install
index b9dd663..eb989f8 100644
--- a/mailsystem.install
+++ b/mailsystem.install
@@ -65,8 +65,12 @@ function mailsystem_update_7300() {
       array('!count' => 0));
   }
 
-  $class_dir = drupal_realpath(file_build_uri('mailsystem')) . '/';
-  $class_dir = substr($class_dir, strlen(DRUPAL_ROOT) + 1);
+  $class_dir = drupal_realpath(file_build_uri('mailsystem'));
+  $class_dir = drupal_realpath(file_build_uri('mailsystem'));
+  if ($class_dir===false) {
+    throw new DrupalUpdateException('The mailsystem class directory could not be located.');
+  }
+  $class_dir = substr($class_dir.'/', strlen(DRUPAL_ROOT) + 1);
   $genclasses = db_select('registry', 'r')
     ->fields('r', array('name', 'filename'))
     ->condition('filename', db_like($class_dir) . '%', 'LIKE')