Spock - Stubbed method is not returning the expected value?

I have the following use case

protected void saveSDAs(SdaCreationRequest request){
    List<SaveSDAReq> saveSDARequests = saveSDAsRequestMapper.map(request);
    for (SaveSDAReq saveSDARequest : saveSDARequests) {
        SaveSDAResponse response = mvasV1Service.saveSDA(saveSDARequest, RETRY);
        LOG.info("response status: {}", response.getSaveSDAResp().getResponseStatus().getStatus());
    }
}

As you can see, in order to test this method in Spock, I would like to stub the Mapper interface to return List<Foo> objects, which is then looped through and passed into Creator interface.

def saveSDAs(){
    given:
    def request = new SdaCreationRequest()

    def creator = new LegacyClientSDACreator()
    def requestMapper = Stub(WebServiceRequestMapper)
    creator.setSaveSDAsRequestMapper(requestMapper)

    def saveSDAReq1 = new SaveSDAReq()
    requestMapper.map(request) >> [saveSDAReq1]

    def mvasV1Service = Mock(MVASV1Service)
    creator.setMvasV1Service(mvasV1Service)

    when:
    creator.saveSDAs(request)

    then:
    1 * mvasV1Service.submitSDA(_, true)
}

However, instead of the stubbed list [saveSDAReq1], it is returning null. What am I doing wrong here?