Java, reflection convert field into JAXBElement

I am trying to perform logging of SOAPMessage. This object contains both wrapper classes and JAXBElements, I am doing something like this

@Before("soapRequest()")
public void logBefore(JoinPoint joinPoint) {

    Object[] signatureArgs = joinPoint.getArgs();
    System.out.println("\n\n\n");
    for (Object signatureArg : signatureArgs) {
        StringBuilder sb = new StringBuilder();

        try {

            Field[] aClassFields = signatureArg.getClass().getDeclaredFields();
            sb.append(signatureArg.getClass().getSimpleName() + " [ ");
            for (Field f : aClassFields) {
                f.setAccessible(true);
                String fName = f.getName();

                String value = "";
                if(f.get(signatureArg) instanceof JAXBElement) {
                    log.info("is instance of");
                    JAXBElement val = (JAXBElement) f.get(signatureArg);
                    log.info(val.toString());
                    value = val.getValue().toString();
                } else {
                    value = f.get(signatureArg).toString();
                }

                sb.append("(" + f.getType() + ") " + fName + " = " + value + ", ");
            }
            sb.append("]");
        } catch (Exception e) {
            e.printStackTrace();
        }

        System.out.println(sb.toString());
    }
}

However this line throws NPE:

if(f.get(signatureArg) instanceof JAXBElement) {
                        log.info("is instance of");
                        JAXBElement val = (JAXBElement) f.get(signatureArg);
                        log.info(val.toString());
                        value = val.getValue().toString();
                    }

How Can I check if the field is instance of JAXBElement and extract value from it?