diff --git a/classes/kohana/orm.php b/classes/kohana/orm.php
index d4f2aaa..299ba93 100644
--- a/classes/kohana/orm.php
+++ b/classes/kohana/orm.php
@@ -681,7 +681,9 @@ public function set($column, $value)
 			$this->_related[$column] = $value;
 
 			// Update the foreign key of this model
-			$this->_object[$this->_belongs_to[$column]['foreign_key']] = $value->pk();
+			$this->_object[$this->_belongs_to[$column]['foreign_key']] = ($value instanceof ORM)
+				? $value->pk()
+				: NULL;
 
 			$this->_changed[$column] = $this->_belongs_to[$column]['foreign_key'];
 		}