Error when creating EntityManager in JPA2

I'm trying to create a simple JPA2 app that persists something to a database using the OpenJPA implementation. I'm running Derby and using the default Embedded JDBC Driver. I've been scratching my head over this trying to figure out what's missing. I'm sure it's something trivial, but if someone could look this over and point me in the right direction, I'd be really greatful. Thanks.

Here's my persistence.xml:

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.0" xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">
    <persistence-unit name="HelloWorld">
        <jta-data-source>java:comp/env/StuffDerby</jta-data-source>
        <class>helloworld.entities.Employee</class>
    </persistence-unit>
</persistence>

and when I run this code:

EntityManagerFactory emf = Persistence.createEntityManagerFactory("HelloWorld");
EntityManager em = emf.createEntityManager();

It fails when calling the createEntityManager() method.

406  HelloWorld  INFO   [WebContainer : 0] openjpa.Runtime - Starting OpenJPA 2.0.0
[1/11/11 12:46:21:377 PST] 0000002f servlet       E com.ibm.ws.webcontainer.servlet.ServletWrapper service SRVE0068E: Uncaught exception created in one of the service methods of the servlet HelloServlet in application HelloWorld. Exception created : <openjpa-2.0.0-r422266:935683 nonfatal general error> org.apache.openjpa.persistence.PersistenceException: java.lang.String incompatible with javax.sql.DataSource
    at org.apache.openjpa.kernel.AbstractBrokerFactory.newBroker(AbstractBrokerFactory.java:208)
    at org.apache.openjpa.kernel.DelegatingBrokerFactory.newBroker(DelegatingBrokerFactory.java:156)
    at org.apache.openjpa.persistence.EntityManagerFactoryImpl.createEntityManager(EntityManagerFactoryImpl.java:213)
    at com.ibm.ws.persistence.EntityManagerFactoryImpl.createEntityManager(EntityManagerFactoryImpl.java:45)
    at com.ibm.ws.persistence.EntityManagerFactoryImpl.createEntityManager(EntityManagerFactoryImpl.java:30)
    at helloworld.entities.controller.EmployeeManager.getEntityManager(EmployeeManager.java:37)
    at helloworld.entities.controller.EmployeeManager.getEmployee(EmployeeManager.java:188)
    at helloworld.HelloServlet.doGet(HelloServlet.java:47)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:718)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:831)
    at com.ibm.ws.webcontainer.servlet.ServletWrapper.service(ServletWrapper.java:1661)
    at com.ibm.ws.webcontainer.servlet.ServletWrapper.service(ServletWrapper.java:1595)
    at com.ibm.ws.webcontainer.filter.WebAppFilterChain.doFilter(WebAppFilterChain.java:131)
    at com.ibm.ws.osgi.web.extender.OsgiDirectoryProtectionFilter.doFilter(OsgiDirectoryProtectionFilter.java:94)
    at com.ibm.ws.webcontainer.filter.FilterInstanceWrapper.doFilter(FilterInstanceWrapper.java:188)
    at com.ibm.ws.webcontainer.filter.WebAppFilterChain.doFilter(WebAppFilterChain.java:116)
    at com.ibm.ws.webcontainer.filter.WebAppFilterChain._doFilter(WebAppFilterChain.java:77)
    at com.ibm.ws.webcontainer.filter.WebAppFilterManager.doFilter(WebAppFilterManager.java:908)
    at com.ibm.ws.webcontainer.servlet.ServletWrapper.handleRequest(ServletWrapper.java:932)
    at com.ibm.ws.webcontainer.servlet.ServletWrapper.handleRequest(ServletWrapper.java:500)
    at com.ibm.ws.webcontainer.servlet.ServletWrapperImpl.handleRequest(ServletWrapperImpl.java:178)
    at com.ibm.ws.webcontainer.webapp.WebApp.handleRequest(WebApp.java:3826)
    at com.ibm.ws.webcontainer.webapp.WebGroup.handleRequest(WebGroup.java:276)
    at com.ibm.ws.webcontainer.WebContainer.handleRequest(WebContain开发者_JAVA技巧er.java:931)
    at com.ibm.ws.webcontainer.WSWebContainer.handleRequest(WSWebContainer.java:1583)
    at com.ibm.ws.webcontainer.channel.WCChannelLink.ready(WCChannelLink.java:186)
    at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.handleDiscrimination(HttpInboundLink.java:455)
    at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.handleNewInformation(HttpInboundLink.java:384)
    at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.ready(HttpInboundLink.java:272)
    at com.ibm.ws.tcp.channel.impl.NewConnectionInitialReadCallback.sendToDiscriminators(NewConnectionInitialReadCallback.java:214)
    at com.ibm.ws.tcp.channel.impl.NewConnectionInitialReadCallback.complete(NewConnectionInitialReadCallback.java:113)
    at com.ibm.ws.tcp.channel.impl.AioReadCompletionListener.futureCompleted(AioReadCompletionListener.java:165)
    at com.ibm.io.async.AbstractAsyncFuture.invokeCallback(AbstractAsyncFuture.java:217)
    at com.ibm.io.async.AsyncChannelFuture.fireCompletionActions(AsyncChannelFuture.java:161)
    at com.ibm.io.async.AsyncFuture.completed(AsyncFuture.java:138)
    at com.ibm.io.async.ResultHandler.complete(ResultHandler.java:204)
    at com.ibm.io.async.ResultHandler.runEventProcessingLoop(ResultHandler.java:775)
    at com.ibm.io.async.ResultHandler$2.run(ResultHandler.java:905)
    at com.ibm.ws.util.ThreadPool$Worker.run(ThreadPool.java:1550)
Caused by: java.lang.ClassCastException: java.lang.String incompatible with javax.sql.DataSource
    at org.apache.openjpa.jdbc.conf.JDBCConfigurationImpl.createConnectionFactory(JDBCConfigurationImpl.java:834)
    at org.apache.openjpa.jdbc.conf.JDBCConfigurationImpl.getDBDictionaryInstance(JDBCConfigurationImpl.java:594)
    at org.apache.openjpa.jdbc.meta.MappingRepository.endConfiguration(MappingRepository.java:1489)
    at org.apache.openjpa.lib.conf.Configurations.configureInstance(Configurations.java:507)
    at org.apache.openjpa.lib.conf.Configurations.configureInstance(Configurations.java:432)
    at org.apache.openjpa.lib.conf.PluginValue.instantiate(PluginValue.java:104)
    at org.apache.openjpa.conf.MetaDataRepositoryValue.instantiate(MetaDataRepositoryValue.java:68)
    at org.apache.openjpa.lib.conf.ObjectValue.instantiate(ObjectValue.java:83)
    at org.apache.openjpa.conf.OpenJPAConfigurationImpl.newMetaDataRepositoryInstance(OpenJPAConfigurationImpl.java:939)
    at org.apache.openjpa.conf.OpenJPAConfigurationImpl.getMetaDataRepositoryInstance(OpenJPAConfigurationImpl.java:930)
    at org.apache.openjpa.kernel.AbstractBrokerFactory.makeReadOnly(AbstractBrokerFactory.java:626)
    at org.apache.openjpa.kernel.AbstractBrokerFactory.newBroker(AbstractBrokerFactory.java:193)
    ... 38 more

org.apache.openjpa.persistence.PersistenceException: 
    java.lang.String incompatible with javax.sql.DataSource

In other words, java:comp/env/StuffDerby points to a java.lang.String instead of java.sql.DataSource. Fix it accordingly in the server config.