Post photo to Tumblr with AS3 (Error #2101 decode())

I'm trying to post a photo to Tumblr.com from Flash but it results in an error #2101

Error #2101: The String passed to URLVariables.decode() must be a URL-encoded query string containing name/value pairs

This is the code I'm using: ( ActionScript3 )

import com.adobe.images.PNGEncoder;
import flash.display.BitmapData;
import flash.display.MovieClip;
import flash.events.Event;
import flash.events.SecurityErrorEvent;
import flash.events.IOErrorE开发者_运维问答vent;
import flash.net.URLLoader;
import flash.net.URLLoaderDataFormat;
import flash.net.URLRequest;
import flash.net.URLRequestMethod;
import flash.utils.ByteArray;
//
var myLoader:URLLoader;
myLoader = new URLLoader();
myLoader.dataFormat = URLLoaderDataFormat.VARIABLES;
//
var urlVars:URLVariables = new URLVariables();
urlVars.email = "***@*****.de";
urlVars.password = "*****";
//
urlVars.type = "photo";
//
var b:BitmapData = new BitmapData(mc.width,mc.height,true);
b.draw(mc);
var png:ByteArray = PNGEncoder.encode(b);
//
urlVars.data = png;
//
var urlRequest:URLRequest = new URLRequest("http://www.tumblr.com/api/write");
//
urlRequest.contentType = "application/octet-stream";
urlRequest.method = URLRequestMethod.POST;
urlRequest.data = urlVars;
myLoader.load(urlRequest)

Any ideas very welcome.

Thanks Bob