This can be fixed easily in BaseCallHandler.marshallInbound() via:
// If a converter is not specified for the Return type than we need to set a MarshalException on the call and
// ensure that this call is not executed per JIRA DWR-322.
Class<?> returnType = method.getReturnType();
if (null != returnType && !converterManager.isConvertable(returnType))
call.setMarshallFailure(new ConversionException(returnType, "A converter has not been specified for the return type of " + method.getName() + " method cannot be executed."));
However, I am not sure this is the best place for it. Thoughts?