iTextSharp: Trying to make TextField transparent: PdfContentByte is nothing - itext

Question

I want to add textfields to my PDF.
These textfields' backgrounds should be transparent.
I have found an example on the web that showed how to do this the following way:
PdfWriter writer = PdfWriter.getInstance(document, new FileOutputStream("TransparencyPDF.pdf"));
document.open();
PdfContentByte cb = writer.getDirectContent();
PdfGState gs1 = new PdfGState();
gs1.setFillOpacity(0.5f);
cb.setGState(gs1);
However, in my code, I don't have a PDFWriter. I have a PDFStamper.
But its properties look perfectly the same, so I adopt it like this:
Dim cb As PdfContentByte = stamper.GetOverContent(0)
Dim gs As New PdfGState
gs.FillOpacity = 0.5
cb.SetGState(gs)
The last line throws the error "System.NullReferenceException: cb is nothing."
What am I doing wrong here?
Thank you.
This is my entire code:
Dim stamper As PdfStamper = New PdfStamper(New PdfReader(sInputFile), File.Create(sOutputFile))
Dim iPageNumer As Integer = 1
Dim tf As TextField
tf = New TextField(stamper.Writer, New iTextSharp.text.Rectangle(iLowerLeftX, iLowerLeftY, iUpperRightX, iUpperRightY), n.Name)
Dim bf = BaseFont.CreateFont(BaseFont.HELVETICA, BaseFont.CP1252, False)
With tf
.Alignment = Element.ALIGN_LEFT And Element.ALIGN_MIDDLE ' Element.ALIGN_CENTER And Element.ALIGN_MIDDLE
.BackgroundColor = GrayColor.WHITE
.BorderColor = Color.RED
.BorderStyle = PdfBorderDictionary.STYLE_SOLID
.DefaultText = "This is a new text field."
.Font = bf
.FontSize = 7
.MaxCharacterLength = 25
.Options = TextField.BORDER_WIDTH_MEDIUM ' TextField.REQUIRED Or TextField.MULTILINE
.Rotation = 0 '90
.Text = "" 'This is the assigned value."
End With
stamper.AddAnnotation(tf.GetTextField(), iPageNumer)
Dim cb As PdfContentByte = stamper.GetOverContent(0)
Dim gs As New PdfGState
gs.FillOpacity = 0.5
cb.SetGState(gs) 'This line throws the error. cb is nothing. Why?
stamper.Close()

Answer 1

The problem with
Dim cb As PdfContentByte = stamper.GetOverContent(0)
is that it requests the OverContent of page 0 while page numbering in iText starts at 1. Thus, use
Dim cb As PdfContentByte = stamper.GetOverContent(1)
instead.