troubled by "SEVERE: Cannot register null bean"

[Rob Sargent]

I'm getting the following error message during startup
   SEVERE: Cannot register null bean for
   
[Tomcat:type=DataSource,host=localhost,context=/sgs,class=javax.sql.DataSource,name="jdbc/sgsdb"]

with sgs/META-INF/context.xml as

   <Context reloadable="true">
      <Resource
        name="jdbc/sgsdb"
        url="jdbc:postgresql://localhost:5432/version4007"
        driverClassName="org.postgresql.Driver"

        // a dozen attribute elided.

   type="javax.sql.DataSource"
   classname="org.apache.tomcat.dbcp.dbcp.datasources.PerUserPoolDataSource"
   factory="org.apache.commons.dbcp2.datasources.PerUserPoolDataSourceFactory"
      />
   </Context>

and sgs/WEB-INF/web.xml as

   <?xml version="1.0" encoding="ISO-8859-1"?>
   <web-app xmlns="http://java.sun.com/xml/ns/j2ee";
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance";
         xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
   http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd";
         version="2.5">
      <display-name>SGSaaS</display-name>
      <context-param>
        <description>Enable debugging for the application</description>
        <param-name>debug</param-name>
        <param-value>true</param-value>
      </context-param>
      <env-entry>
        <env-entry-name>databaseHost</env-entry-name>
   <env-entry-value>${SGSSRVR_databaseHost}</env-entry-value>
   <env-entry-type>java.lang.String</env-entry-type>
      </env-entry>
      <env-entry>
        <env-entry-name>databasePort</env-entry-name>
   <env-entry-value>${SGSSRVR_databasePort}</env-entry-value>
   <env-entry-type>java.lang.Integer</env-entry-type>
      </env-entry>
      <env-entry>
        <env-entry-name>roleExtension</env-entry-name>
   <env-entry-value>${SGSSRVR_roleExtension}</env-entry-value>
   <env-entry-type>java.lang.String</env-entry-type>
      </env-entry>
      <env-entry>
        <env-entry-name>expansionStep</env-entry-name>
   <env-entry-value>${SGSSRVR_expansionStep}</env-entry-value>
   <env-entry-type>java.lang.Integer</env-entry-type>
      </env-entry>
      <resource-ref>
        <description>Pointer to context datasource</description>
        <res-ref-name>jdbc/sgsdb</res-ref-name>
        <res-type>javax.sql.DataSource</res-type>
        <res-auth>Container</res-auth>
      </resource-ref>
   </web-app>


Deep down in NamingContextListen

   actualResource =envCtx.lookup(resource.getName());

fails, setting actualResource to null and resource.getName() is "jdbc/sgsdb"

I can get the env values (some of which are superfluous) in my app, but 
I cannot get the DataSource I think because it isn't there.

I've tried (with castings removed for brevity)

   ctx = initialContext.lookup("java:/comp/env");
   dataSource = ctx.lookup("jdbc/sgsdb");

   dataSource=initialContext.lookup("sgs/jdbc/sgsdb") //with and
   without leading slash

and several permutations of these.

Where have a gone afoul of the XMLgod(s)?