--- a/services/sync/modules/stores.js
+++ b/services/sync/modules/stores.js
@@ -163,17 +163,18 @@ SnapshotStore.prototype = {
       // special-case guid changes
       let newGUID = command.data.GUID,
       oldGUID = command.GUID;
 
       this._data[newGUID] = this._data[oldGUID];
       delete this._data[oldGUID];
 
       for (let GUID in this._data) {
-        if (this._data[GUID].parentGUID == oldGUID)
+        if (("parentGUID" in this._data[GUID]) &&
+            (this._data[GUID].parentGUID == oldGUID))
           this._data[GUID].parentGUID = newGUID;
       }
     }
     for (let prop in command.data) {
       if (prop == "GUID")
         continue;
       this._data[command.GUID][prop] = command.data[prop];
     }