VectorWriter extends java.io.PrintWriter but it does not suppply its forgiving 
services when trying to println(null). Each of Sun's implementations of println
(Object) is capable of handling the singular case of Object==null gracefully, 
but ThrowableInformation.VectorWriter does not, which effectively breaks custom 
made printStackTrace methods at the very instant, when the stack trace is to be 
output to a logger.

In detail, org.apache.log4j.spi.ThrowableInformation.VectorWriter.println
(Object) looks as follows:

  public void println(Object o) {      
    v.addElement(o.toString());
  }

whilst it might preferrably be:

  public void println(Object o) {      
    v.addElement(String.valueOf(o));  
  }