diff --git a/app/models/Data/User.php b/app/models/Data/User.php
index 86a8707..1135372 100644
--- a/app/models/Data/User.php
+++ b/app/models/Data/User.php
@@ -4,7 +4,7 @@ namespace App\Model\Data;
 
 use Phalcon\Mvc\Model,
     Phalcon\Validation,
-    Phalcon\Validation\Validator\Uniqueness,
+    Phalcon\Validation\Validator\Callback as CallbackValidator,
     InvalidArgumentException,
     Httpcb\OAuth\UserData\UserDataInterface;
 
@@ -52,8 +52,14 @@ class User extends Model
         // Validation
         $validator = new Validation();
 
-        $validator->add('username', new Uniqueness(['message' => 'The username already exists.']));
-        $validator->add('email', new Uniqueness(['message' => 'The email address already exists.']));
+        $validator->add('username', new CallbackValidator([
+            'callback' => function() { return $this->findFirstByUsername($this->getUsername()) === false; },
+            'message' => 'The username already exists.'
+        ]));
+        $validator->add('email', new CallbackValidator([
+            'callback' => function() { return $this->findFirstByEmail($this->getEmail()) === false; },
+            'message' => 'The email address already exists.'
+        ]));
 
         return $this->validate($validator);
     }