Invisible Signature should have empty Appearance Dictionary

private void prepareNonVisibleSignature(PDSignatureField signatureField)
        throws IOException
{
    // "Signature fields that are not intended to be visible shall
    // have an annotation rectangle that has zero height and width."
    // Set rectangle for non-visual signature to rectangle array [ 0 0 0 0 ]
    signatureField.getWidgets().get(0).setRectangle(new PDRectangle());
+    signatureField.getWidgets().get(0).setAppearance(new PDAppearanceDictionary());
}