JS MalformedMessage JSON syntax err

I’m implementing a js and trying to catch all the different errors which can occur when processing a JoinReq. I catch all cases with appropriate result codes but there is one case which seems a little ambigious / troublesome, at least to me. That is, the case where the JSON can’t get parsed due to syntax errors. Should status code 4xx or 5xx be returned in this case or 200 with ResultCode “MalformedMessage”. Also, in case of http response code 200 - the mandatory fields in JoinAns properly can’t be occupied due to the corrupt JoinReq which would trigger a ErrorNotif from receiving NS. I don’t know if my take is compeltely wrong or if I have missed something in the spec ?

Note: I’m not talking about the cases where a mandatory field is missing, or the lengths or contents of the JoinReq are incorrect, but the cases where the parsing of JoinReq simply fails because of incorrect syntax.