diff --git a/models/User.js b/models/User.js
index 8733d9b84d..2d862c3271 100644
--- a/models/User.js
+++ b/models/User.js
@@ -4,7 +4,7 @@ var mongoose = require('mongoose'),
 var userSchema = new mongoose.Schema({
 
   // Local authentication
-  username: { type: String, index: true, unique: true },
+  username: { type: String, unique: true, sparse: true },
   password: String,
 
   // OAuth 2.0 authentication