error running portlet

Hi All,

I'm deploying a portlet based on the Hepper's HelloWorld example book.
The deployment happens fine but when the portlet is shown, I get the
following error on the browser:

HelloWorld
HelloWorld  view  |  max   min   nor
Error occurred in portlet!

And in the log files I get:

SEVERE: Error in Portlet
java.lang.NullPointerException
	at org.apache.pluto.invoker.impl.PortletInvokerImpl.invoke(PortletInvokerImpl.java:109)
	at org.apache.pluto.invoker.impl.PortletInvokerImpl.load(PortletInvokerImpl.java:80)
	at org.apache.pluto.PortletContainerImpl.portletLoad(PortletContainerImpl.java:218)
	at org.apache.pluto.portalImpl.core.PortletContainerWrapperImpl.portletLoad(PortletContainerWrapperImpl.java:94)
	at org.apache.pluto.portalImpl.aggregation.PortletFragment.service(PortletFragment.java:120)
	at org.apache.jsp.WEB_002dINF.aggregation.ColumnFragment_jsp._jspService(org.apache.jsp.WEB_002dINF.aggregation.ColumnFragment_jsp:65)
	at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
	at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:322)
	at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:291)
	at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:241)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
	at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:672)
	at org.apache.catalina.core.ApplicationDispatcher.doInclude(ApplicationDispatcher.java:574)
	at org.apache.catalina.core.ApplicationDispatcher.include(ApplicationDispatcher.java:499)
	at org.apache.pluto.portalImpl.aggregation.AbstractFragment.service(AbstractFragment.java:112)
	at org.apache.jsp.WEB_002dINF.aggregation.RowFragment_jsp._jspService(org.apache.jsp.WEB_002dINF.aggregation.RowFragment_jsp:65)
	at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)

Any ideas? I'm completely lost.

Thanks,
Daniel