Grails Test Domain Model - Mocks

I am trying to test some methods within a Domain object, the code seems to execute (based on log) but the assertions fail. I assume I am doing something wrong in the mocking. I am including a simplified version of my code. Based on the log output, the assertion should pass.

class EventDate{
   Date start
   Date end
}

class Appointments {
     hasMany = [ eventDates: EventDate]

     def extendDates(start,end){
         //some logic on eventDates...
         if(create_new) EventDate(start,end).save(flush:true);
     }
}

import org.junit.*
import grails.test.mixin.*

@TestFor(Appointments)
@Mock([EventDate,Appointments])
class AppointmentsTests {

    def testDomain 

    void setUp(){
        testDomain = new Appointments()
        mockDomain(Appointments, [testDomain])
    }
    void testCoreExtend(){
        //Log indicates the correct execution and creation of event
        testDomain.extendDates(new Date(),null)
        //following returns NullPointerException
        assertTrue testDomain.eventDates.size() == 1
    }
}