diff --git a/lib/database.js b/lib/database.js
index b2406d0..d075711 100644
--- a/lib/database.js
+++ b/lib/database.js
@@ -447,7 +447,8 @@ Database.prototype.enforceUniqueness = function(collectionName, values, updatedI
     if(!attrDef.unique) continue;
 
     for (var index in this.data[collectionName]) {
-
+      if (typeof this.data[collectionName][index] === "undefined" || this.data[collectionName][index] === null) continue;
+      
       // Ignore uniqueness check on undefined values
       // (they shouldn't have been stored anyway)
       if (_.isUndefined(values[attrName])) continue;