FixedContentEditor has invalid arguments

Hi, I am new to PDFProcessing, was following the example codes, but could not compile...Error 2 Argument 1: cannot convert from 'Telerik.Windows.Pdf.Documents.Fixed.Model.RadFixedPage' to 'Telerik.Windows.Documents.Fixed.Model.Common.IContentRootElement' 

Error 1 The best overloaded method match for 'Telerik.Windows.Documents.Fixed.Model.Editing.FixedContentEditor.FixedContentEditor(Telerik.Windows.Documents.Fixed.Model.Common.IContentRootElement)' has some invalid arguments 
 

I am using version 2015.1.331.40, runtime version v4.0.30319

RadFixedDocument document = new RadFixedDocument();
RadFixedPage page = document.Pages.AddPage();
FixedContentEditor editor = new FixedContentEditor(page);
editor.DrawText("Hello RadPdfProcessing!");
PdfFormatProvider provider = new PdfFormatProvider();
using (Stream output = File.OpenWrite("Hello.pdf"))
{
provider.Export(document, output);
}