Why will session.parseBody() function cause null error when I lanuch it for the second time in Android?

Question

I use NanoHTTPD as web server in my Android App, it's OK when I lanuch session.parseBody() for the first time.

But it cause null error when I lanuch the session.parseBody() function for the second time,why?

Is there some bugs in the function session.parseBody()?

@Override
public Response serve(IHTTPSession session) {

    Method method = session.getMethod();
    if (Method.POST.equals(method)) { 

         Map<String, String> files = new HashMap<String, String>();

         session.parseBody(files); //It's OK
         String zipValue1=session.getParms().get("ActionZIPMultiple");
         Utility.LogError("KK1: "+zipValue1);

         session.parseBody(files); //It will cause error!
         String zipValue2=session.getParms().get("ActionZIPMultiple");
         Utility.LogError("KK2: "+zipValue2);
     }

}