Failed transformation of value at keyPath 'TEST_ANSWER'. No strategy for transforming from '__NSDictionaryM' to 'NSString'

[Aet]

I got the following error 

restkit.object_mapping:RKObjectMappingOperation.m:244 Failed transformation of value at keyPath 'QUESTION_ANSWER'. No strategy for transforming from '__NSDictionaryM' to 'NSString'

But for all other xml fields it just works. I saw in the xml file, that the question_answer is sometimes null and the xml looks in this case like that:

<TEST_ANSWER><TEST_ANSWER/>

Is that probably a problem? How can I fix this?