Grails 2.4.4 : How do mock a transient service inside a domain?

I have been trying to figure this out for 2 days now and I am really stuck and frustrated. I have a domain object with a service which is being used for custom validation. The domain looks like this:

class Llama { 
String name
transient myFetcherService

static transients = [
            'myFetcherService'
    ]

static constraints = {
        name validator: { val, obj ->
            if (obj.nameExists(val) == true) {
                //return some error here.
            }
        }
    }

protected boolean nameExists(String name) {
        List<Llama> llamasList = myFetcherService.fetchExistingLlamasByName(name)

        if (llamasList.isEmpty()) {
            return false
        }

        return true
    }
}

Now, I have another Service, which simply saves a list of Llama objects. It looks like this:

class LlamaFactoryService {

   public void createLlamas(List<String> llamaNames) {
     llamaNames.each { name ->
         new Llama(name: name).save()
     }
   }

}

In my test. I keep getting this error:

Failure:  createLlamas should create Llammas (com.myLlamaProject.LlamaFactoryServiceSpec)
|  java.lang.NullPointerException: Cannot invoke method myFetcherService on null object

I don't understand. In my tests, added a metaClass for the service in the "given" section. When it tries to save, it's telling that the service is null. This is what my test looks like:

given:
def myFetcherService = mockFor(MyFetcherService)   
myFetcherService.demand.fetchExistingLlamasByName {def name -> return []}
Llama.metaClass.myFetcherService = myFetcherService.createMock()

when:
service.createLlamas(['Pablo','Juan','Carlos'])

then:
//some validations here....

I also tried using metaClass on the method nameExists(), but it gives me the same nullPointerException as the one above. Could someone point me to the right direction? I'm a bit stuck. :(

Thanks in advance for reading and helping.