$cordovaFileTransfer.download returns "null" always

{"code":null,"source":null,"target":null,"http_status":null,"body":null,"exception":null}

         var url = "http://foo.bar/img/image.png";
         var filename = url.split("/").pop();
         var targetPath = cordova.file.externalRootDirectory + 'Pictures/' + filename;


         console.log(filename); // returns "image.png"
         console.log(targetPath); // returns "file:///storage/emulated/0/Pictures/image.png"

            $cordovaFileTransfer.download(url, targetPath, {}, true).then(function (result) {
                  console.log(JSON.stringify(result));
                  $scope.hasil = 'Save file on '+targetPath+' success!';
                  $scope.mywallpaper=targetPath;
            }, function (error) {
/* return {"code":null,"source":null,"target":null,"http_status":null,"body":null,"exception":null}*/
                  console.log(JSON.stringify(error));
                  $scope.hasil = 'Error Download file';
            }, function (progress) {
                  $scope.downloadProgress = (progress.loaded / progress.total) * 100;
            });