a problem to add an Xpath assertion to a rest request

I am now doing rest test by using soapui. In this case, i post some thing to server. If there is no error by request, the server will send a response in XML back, as following:



if there is some wrong, the response will be

Place not found for placeId2000

So, i wanna insert such an Xpath assertion, 
//data[@contentType="null"]
that means, if the contentType is "null", then the request/response are right, otherwise, wrong.

But this assertion is always wrong, showed as "null/empty response"
and i think is a problem by

Does anyone has a good idea to ignore or maybe solve that problem.

Thanks in advance!!