diff --git a/lib/Relationship.php b/lib/Relationship.php
index 297d89438..756ea038f 100644
--- a/lib/Relationship.php
+++ b/lib/Relationship.php
@@ -540,7 +540,7 @@ private function get_foreign_key_for_new_association(Model $model)
 		$primary_key = Inflector::instance()->variablize($this->foreign_key[0]);
 
 		return array(
-			$primary_key => $model->id,
+			$primary_key => $model->{$primary_key},
 		);
 	}