ClassNotFoundException with writeReplace() classes

We're using XStream to pass some objects from sever-side back to the client. The server has some internal classes that only exist server-side, but during serialization they're writeReplace()'ed to a common API class. When the xml gets deserialized by XStream client-side it sees something like this:
...
<variable class="com.server.$$InternalClass$$" resolves-to="com.api.NormalClass">
...

The class 'com.server.$$InternalClass$$' doesn't exist client-side, but 'com.api.NormalClass' does. This shouldn't cause a problem, but XStream is still throwing a ClassNotFoundException.

I've attached a zip file containing two files:

• WriteReplaceTest.java - a test case demonstrating the bug with a contrived example.
• xstream-1.3-FB10536.patch - a simple patch that fixed this problem for our particular case. I'm hoping there's a better, more general solution.