Facebook Android request error : GraphObject is null -

i'm trying send simple request android facebook sdk. here code :

session.openactivesession( native_activty, true, new session.statuscallback()  {     @override     public void call(session session, sessionstate state, exception exception)      {         request.callback callback= new request.callback()          {             public void oncompleted(response response)             {                 if( response.geterror() != null)                 {                     log.i( "output", response.tostring());                     // here i'm checking token, permissions, ...                 }             }         };          bundle params = new bundle();          params.putstring("access_token", session.getaccesstoken());         params.putstring("name", name);         params.putstring("caption", caption);         params.putstring("description", description);         params.putstring("link", link);         params.putstring("picture", picture);          request request = new request( session, "me/feed/", params, httpmethod.post, callback );         request.executeasync();               log.i( "output", request.tostring() );     } } 

but request fails, , response :

i/output(29990): {response:  responsecode: 200, graphobject: null, error: {httpstatus: -1, errorcode: -1, errortype: null, errormessage: null}, isfromcache:false} 

with error

i/output(29990): com.facebook.facebookexception: not construct request body 

i have checked several things :

• permissions ok;
• access_token valid (https://graph.facebook.com/me?access_token={my_token} works fine browser)
• the session state opened

i don't understand why doesn't works, , don't know do...

many thanks