-

diff --git a/data/model/Relationship.php b/data/model/Relationship.php
index 0e9c184..77619a5 100644
--- a/data/model/Relationship.php
+++ b/data/model/Relationship.php
@@ -208,7 +208,7 @@ class Relationship extends \lithium\core\Object {
 		$conditions = (array) $this->constraints();
 
 		foreach ($this->key() as $from => $to) {
-			if (!isset($object->{$from})) {
+			if ($object->{$from} === null) {
 				return null;
 			}
 			$conditions[$to] = $object->{$from};