iText - java.lang.RuntimeException: Unbalanced save/restore state operators - itext

Question

I receive the error message "java.lang.RuntimeException: Unbalanced save/restore state Operators", when I try to use setClip(...) several times during drawing with a Graphics2-object, which I got from iText's cb.createGraphics(..).
Additional Information:
single threaded application
error after 10 calls to setClip()
allways a pair setClip(path) and setClip(null)
occurs with version 2.1.4; with Version 2.1.7 the error even appears earlier
without the setClip(...) everything is drawn as expected without errors
Does anybody know a solution for this problem, or has a hint to it?
Does this error appear in Version 5..., too? (Before I buy a licence for version 5 ;-)
Thank you!
FileOutputStream fos = new FileOutputStream("c:\\temp\\test.pdf");
Document document = null;
Rectangle rect=PageSize.A3.rotate();
document = new Document(rect);
PdfWriter writer=PdfWriter.getInstance(document, fos);
document.open();
PdfContentByte cb = writer.getDirectContent();
Graphics2D g =cb.createGraphics(rect.getWidth(), rect.getHeight());
int x[]={10,1000,1000,10};
int y[]={10,10,1000,1000};
Polygon po=new Polygon();
for (int i=0; i<4; i++)
po.addPoint(x[i],y[i]);
g.clip(po);
g.setClip(null);
document.newPage();
g.clip(po);
g.setClip(null);