The writeNamespace method is defined as follows:

  /**
   * Writes a namespace to the output stream
   * If the prefix argument to this method is the empty string,
   * "xmlns", or null this method will delegate to writeDefaultNamespace
   *
   * @param prefix the prefix to bind this namespace to
   * @param namespaceURI the uri to bind the prefix to
   * @throws IllegalStateException if the current state does not allow Namespace writing
   * @throws XMLStreamException
   */
  public void writeNamespace(String prefix, String namespaceURI)
    throws XMLStreamException;


But the code isn't checking for prefix=="xmlns", and hence when it happens it produces malformed documents.