Windows 8 - Windows.Storage WriteTextAsync exception

I want to use StorageFile, to implement Reading&Writing as seen here

http://msdn.microsoft.com/en-us/libr...ting_to_a_file

With

 public async void Save()
 {
     StorageFolder storageFolder = KnownFolders.DocumentsLibrary;
     StorageFile sampleFile = await storageFolder.CreateFileAsync(PlayerPrefs.GetString("WorldName") +"_"+filename_blocks);
     await Windows.Storage.FileIO.WriteTextAsync(sampleFile, "Swift as a shadow");
 }

But the "await" line gives the runtime exception

 Cannot evaluate expression because a native frame is on top of the call stack.} System.Exception {System.UnauthorizedAccessException}

Anyone know what the problem is here?