itextsharp checkbox has value but is not visible as checked

i want to edit my pdf file using itextsharp dll. I have a raido button and checkbox below. I would to have following: if I put a check mark on radio button then checkbox must be visible as checked.

In code radio button has value but when i first time open pdf file checkbox is not checked. when I again put a check mark in the radio button then it is wel visible.

Does anyone have experience with this?

Hier is my code;

PdfReader pdfReader = new PdfReader(pdfTemplate);
            PdfStamper pdfStamper = new PdfStamper(pdfReader, new FileStream(newFile, FileMode.OpenOrCreate)); 
AcroFields pdfFormFields = pdfStamper.AcroFields;
pdfFormFields.SetField("4a.0", "1"); // radio button
pdfFormFields.SetField("4a.1", "1"); // checkbox
pdfFormFields.SetField("4a.2", "2010"); // text box