[TYPO3-mvc] Problem with [Model].subclasses and saving

[Stefan Frömken]

Hello MVC-List,

I have an answer table (tx_kequestionnaire_domain_model_answer) and a 
model (Tx_KeQuestionnaire_Domain_Model_Answer) with all the getter and 
setter methods for the fields in my answer table.
Till now everything works fine.

Now we have the case, that we have different answer types. A good start 
to use the subclasses feature of extbase:

# Answertypes
Tx_KeQuestionnaire_Domain_Model_Answer {
	subclasses {
		Radiobutton = Tx_KeQuestionnaire_Domain_Model_AnswerType_Radiobutton
		Checkbox = Tx_KeQuestionnaire_Domain_Model_AnswerType_Checkbox
	}
}
Tx_KeQuestionnaire_Domain_Model_AnswerType_Radiobutton {
	mapping {
		tableName = tx_kequestionnaire_domain_model_answer
		recordType = Tx_KeQuestionnaire_Domain_Model_AnswerType_Radiobutton
	}
}
Tx_KeQuestionnaire_Domain_Model_AnswerType_Checkbox {
	mapping {
		tableName = tx_kequestionnaire_domain_model_answer
		recordType = Tx_KeQuestionnaire_Domain_Model_AnswerType_Checkbox
	}
}

Till now everything works perfectly. I have different models in my 
result set.

Now comes the problem:
When I want to save my result I have a relation from the result model to 
my answer model:

/**
  * Sets the answer
  *
  * @param Tx_KeQuestionnaire_Domain_Model_Answer $answer
  * @return void
  */
public function setAnswer(Tx_KeQuestionnaire_Domain_Model_Answer $answer) {
	$this->answer = $answer;
}

As long as I don't work with subclasses this method works. But now 
extbase tolds me:

PHP Catchable Fatal Error: Argument 1 passed to 
Tx_KeQuestionnaire_Domain_Model_ResultAnswer::setAnswer() must be an 
instance of Tx_KeQuestionnaire_Domain_Model_Answer, instance of 
Tx_KeQuestionnaire_Domain_Model_AnswerType_Radiobutton given

So...what kind of type I have to add in my setter method?

Stefan