diff --git a/migrations/preen_cron.js b/migrations/preen_cron.js
index 4d141d3ed6..ec1d71bb9c 100644
--- a/migrations/preen_cron.js
+++ b/migrations/preen_cron.js
@@ -37,7 +37,8 @@ db.users.find({
  * Remove empty parties
  */
 db.groups.remove({
-    $where: function(){ return this.type === 'party' && this.members.length === 0; }
+    'type': 'party',
+    $where: "return this.members.length === 0"
 });
 
 /**