Converting page to png creates an empty image

import java.awt.image.BufferedImage;
import java.io.File;

import org.apache.pdfbox.pdmodel.PDDocument;
import org.apache.pdfbox.util.PDFImageWriter;

public class Test {
  public static void main(String[] args) throws Exception {
    PDDocument docOk = PDDocument.load(new File("pdf-ok.pdf"));
    PDDocument docNok = PDDocument.load(new File("pdf-nok.pdf"));

    PDFImageWriter imagewriter = new PDFImageWriter();
    try {
      imagewriter.writeImage(docOk, "png", null, 1, 1, "pdf-ok",
BufferedImage.TYPE_INT_RGB, 100);
      imagewriter.writeImage(docNok, "png", null, 1, 1, "pdf-nok",
BufferedImage.TYPE_INT_RGB, 100);
    } finally {
      docOk.close();
      docNok.close();
    }
  }
}