How can I tell Struts 2 to throw/log exceptions for invalid OGNL Expressions

I have the following class:

public class TestAction {

    public static String concat(String str1, String str2) {
        return str1 + " and " + str2;
    }
}

In a JSP, I can write the following and it works fine:

<s:property value="@org.appfuse.web.TestAction@concat('1', '2')"/>

However, if I change it to an invalid class name, it fails silently:

<s:property value="@org.appfuse.web.InvalidClass@concat('1', '2')"/>

I've tried turning up the logging for org.apache.struts2, com.opensymphony.xwork2 and ognl - but it still fails silently. Any ideas how to get better error messages about invalid OGNL expressions?

Thanks,

Matt