lib/Doctrine/Connection/UnitOfWork.php

@@ -193,9 +193,13 @@ class Doctrine_Connection_UnitOfWork extends Doctrine_Connection_Module
 
             if ($record->hasReference($alias)) {
                 $obj = $record->$alias;
+                
+                // check that the related object is not Doctrine_Null
+                if ($obj instanceof Doctrine_Record) {
                     $obj->save($conn);
                 }
             }
+        }
 
         // save the MANY-TO-MANY associations
         $this->saveAssociations($record);