When you have a new unpersisted object and you call a method to a referenced object - when calling the unlink() method on referenced object the above error is called.

Warning: Illegal offset type in sfDoctrinePlugin/lib/vendor/doctrine/Doctrine/Record.php on line 2459

eg

$foo = new foo();

$foo->getBar(); This call will bind a reference of Bar against the foo object.

$foo->unlink('bar');

As no objects are yet persisted, the unlink method tries to match ids on the objects that do not exist.