Class cast exception when fetching EJB

This code:

Object o = ctx.lookup("java:comp/env/ejb/AdderEJB");
AdderLocalHome home = (AdderLocalHome)o;

generates a ClassCastException. The object returned from the JNDI lookup is something like com.example.ejb.AdderLocalHomeImpl0@1374bef, and I'm trying to cast it to the interface com.example.ejb.AdderLocalHome.

The local reference to the EJB is as follows:

<server-component-ref>
		<description>
		</description>
		<name>ejb/AdderEJB</name>
		<type>interface</type>
		<jndi-name>localejbs/example.com/test-ear/AdderBean</jndi-name>
</server-component-ref>

Why am I getting a casting exception? I believe I'm following the documented methods correctly, but I must be doing something wrong.

BTW, this EJB has also been exposed via a web service, and that functions correctly, so I know the EJB is correctly configured and deployed.

Ideas?