rmock mock object only returns default value

rMock with JUnit 3.81 and Java 1.4.2

I have a implemented method of an interface that changes the ACL of an
object in a document management system:

void changeACL(String docbase, IDfSysObject reference, String
aclDomain, String aclName) throws DfServiceException

I have a test method:

    public final void testChangeACL() {
        /* Set up Mock objects */
        IDfSysObject mockObj = (IDfSysObject) mock(IDfSysObject.class);
        IDoctypeSBO sbo = (IDoctypeSBO) mock(IDoctypeSBO.class);

        /* Expectations */
        try {
            sbo.changeACL("docbasename", mockObj, "newdomain", "newacl");
            mockObj.getACLName();
            mockObj.getACLDomain();
        } catch (DfException dfe) {
        }

        /* Verification */
        startVerification();
        try {
        sbo.changeACL("docbasename", mockObj, "newdomain", "newacl");
            assertThat(mockObj.getACLName(), is.eq("newacl"));
            assertThat(mockObj.getACLDomain(), is.eq("newdomain"));
        } catch (DfException dfe) {
        }
    }

The problem is that the assertions fail with:

com.agical.rmock.core.exception.RMockAssertionFailedException:
ASSERTION FAILED!
<null>
does not pass the expression:
<eq(<newacl>)>
....

I think this is because mockObj.getACLName() is returning null because
it is not affected by the changeACL method. How can I make the
changeACL method have an affect, i.e. change attributes of the mocked
object?

thanks,
-Mark