Added check to ensure a foreign object is actually a Doctrine_Record before calling isModified.

lib/Doctrine/Connection/UnitOfWork.php

@@ -265,7 +265,8 @@ class Doctrine_Connection_UnitOfWork extends Doctrine_Connection_Module
                     $obj = $record->get($rel->getAlias());
 
                     // Protection against infinite function recursion before attempting to save
-                    if ($obj->isModified()) {
+                    if ($obj instanceof Doctrine_Record &&
+                        $obj->isModified()) {
                         $obj->save($this->conn);
                     }
                 }