lock document after sign with itext

Question

after signing a pdf, I want to lock the document, so no more signatures can be added,

I'm trying to add the lock like this:

        PdfDictionary dic2 = new PdfDictionary();
        dic2.put(PdfName.CONTENTS, new PdfString(placeHolder).setHexWriting(true));

        dic2.put(PdfName.LOCK, stamper.getWriter()
                .addToBody(new PdfSigLockDictionary(LockPermissions.NO_CHANGES_ALLOWED)).getIndirectReference());

        pdfAppearance.close(dic2);

but I'm getting this error:

java.lang.IllegalArgumentException: The key /Lock didn't reserve space in preClose().

I can't find a clear example on how to add this lock, what am I doing wrong? I'm using itext 5.

EDIT 1: setting the LOCK lenght in the preclose like this:

PdfSigLockDictionary dic3 = new PdfSigLockDictionary(LockPermissions.NO_CHANGES_ALLOWED);

HashMap<PdfName, Integer> exc = new HashMap<PdfName, Integer>();

exc.put(PdfName.LOCK, dic3.length()* 2 + 2);

pdfAppearance.preClose(exc);

I get no error, buy i see no changes in my pdf: