Extbase MVC Framework

Seems not to work as expected

class Tx_Geniale_Domain_Model_Person extends Tx_Extbase_Domain_Model_FrontendUser {
}

Now I'd like to add a validation on 'email', which is provided in 'Tx_Extbase_Domain_Model_FrontendUser'.

a simple test

 /**
  * @var string
  * @validate EmailAddress
  */
  protected $email;

results in:

#1255072587: The form field name "tx_geniale_pi1[person]" collides with a previous form field name which declared the field as array. (Array overridden by String)