diff --git a/node_revision_diff_field.module b/node_revision_diff_field.module
index 262fb52..4bdf719 100644
--- a/node_revision_diff_field.module
+++ b/node_revision_diff_field.module
@@ -20,7 +20,9 @@ function node_revision_diff_field_field_formatter_info() {
   return array(
     'node_revision_diff_default' => array(
       'label' => t('Comment diff'),
-      'field types' => 'node_revision_diff',
+      'field types' => array(
+        'node_revision_diff',
+      ),
     ),
   );
 }