ClassCastException on Tomcat JNDI lookup

Hello,

I'm trying to set up an AS400JDBCConnectionPoolData Source on Tomcat,
but it returns me this bloody ClassCastException.
The code I'm using to wake up the data source is the following:

Context envCtx = (Context) new
InitialContext().lookup("java:comp/env");
AS400JDBCConnectionPoolDataSource ds = 
    (AS400JDBCConnectionPoolDataSource)envCtx.lookup("jdbc/genbra");
Connection conn = ds.getConnection();


The context has the following settings:
        <Context className="org.apache.catalina.core.StandardContext"
cachingAllowed="true"
charsetMapperClass="org.apache.catalina.util.CharsetMapper"
cookies="true" crossContext="true" debug="1" 
docBase="C:\ProgramFiles\Apache Group\Tomcat 4.1\webapps\JNDItest"
mapperClass="org.apache.catalina.core.StandardContextMapper"
path="/jndi" privileged="false" reloadable="true"
swallowOutput="false" useNaming="true"
wrapperClass="org.apache.catalina.core.StandardWrapper">
          <Resource name="jdbc/genbra" scope="Shareable"
type="javax.sql.DataSource"/>
          <ResourceParams name="jdbc/genbra">
            <parameter>
              <name>maxWait</name>
              <value>5000</value>
            </parameter>
            <parameter>
              <name>maxActive</name>
              <value>4</value>
            </parameter>
            <parameter>
              <name>password</name>
              <value>password</value>
            </parameter>
            <parameter>
              <name>url</name>
              <value>genbra</value>
            </parameter>
            <parameter>
              <name>driverClassName</name>
 <value>com.ibm.as400.access.AS400JDBCConnectionPoolDataSource</value>
            </parameter>
            <parameter>
              <name>maxIdle</name>
              <value>2</value>
            </parameter>
            <parameter>
              <name>username</name>
              <value>username</value>
            </parameter>
          </ResourceParams>
        </Context>

I've read in the IBM program development FAQ that this error is
because the requred jt400.jar is not on the client application
classpath. But I have the jt400.jar file deployed in the
Tomcat\common\lib library.
I've tried several things
1 - To remove the jt400.jar file from classpath, then I received a
ClassNotFoundException
2 - If I specified a non-existing name in the context lookup, it
returned a NamingException that the name is not bound to the context.
So it seems that the context is constructed well, but there is an
error by generating the AS400JDBCConnectionPoolDataSource


What else can be wrong here?

Thank you for your answer

Balazs