[TYPO3-mvc] Extbase validation error

[Thomas "Thasmo" Deinhamer]

Hi guys and gals!

That's my method:

/**
  * Add action for this controller.
  *
  * @param Tx_HypeStore_Domain_Model_Product $product
  * @param integer $quantity
  * @return void
  */
public function addAction(Tx_HypeStore_Domain_Model_Product $product, 
$quantity = NULL) {
// ...
}

The problem is, if quantity is not set at all in the URL,
the following error pops up:
An error occurred while trying to call 
Tx_HypeStore_Controller_CartController->addAction(). Error: Validation 
errors for argument "quantity".

Any idea what's causing this? Is it a problem of Extbase in general
or is it related to Extbase 1.2, which I'm using?

Thanks for any hints on this!

Regards,
Thomas