diff --git a/backbone.js b/backbone.js
index deedb0ed..5476b6a1 100644
--- a/backbone.js
+++ b/backbone.js
@@ -511,8 +511,10 @@
         if (!(model = models[i] = this._prepareModel(models[i], options))) {
           throw new Error("Can't add an invalid model to a collection");
         }
-        if (cids[cid = model.cid] || this._byCid[cid] ||
-          (((id = model.id) != null) && (ids[id] || this._byId[id]))) {
+        
+        cid = model.cid;
+        id = model.id;
+        if (cids[cid] || this._byCid[cid] || (id != null && (ids[id] || this._byId[id]))) {
           throw new Error("Can't add the same model to a collection twice");
         }
         cids[cid] = ids[id] = model;