Spock add mock element to a SortedSet - unit-testing

Question

I have a Spy for an instance of a domain class ProjectSegmentCategory, which contains a SortedSet of instances of another domain class ProjectSegmentCategoryValue. As the two domain classes are both quite complex and have a lot of dependencies, I want to mock both as Mock or Spy.
My problem is, that I don't know, how to add a Spy/Mock to a SortedSet. From the error I get, I look like when I want to add the object to the SortedSet, it tries to create a new object with a Copy constructor and this fails, because the original object is a Spy.
So my question is, how can I add a Spock Spy to a Set?
This is my code:
class ProjectSegmentCategoryGsonSpec extends Specification implements JsonViewTest {
// Define the spy objects needed for the test
def projectsegmentcategory = Spy(ProjectSegmentCategory);
def librarycategory = Spy(LibrarySegmentCategory);
def projectsegmentcategoryvalue = Spy(ProjectSegmentCategoryValue)
Set<ProjectSegmentCategoryValue> projectsegmentcategoryvalues;
def projectsegmentclassification = Spy(ProjectSegmentClassification)
def librarycategoryvalue = Spy(LibrarySegmentCategoryValue)
def setup() {
// Set attributes of the objects for the test
projectsegmentcategory.id = 1;
projectsegmentcategory.hasData = true;
projectsegmentcategory.isClustered = true;
projectsegmentcategory.isUnlockedForBudgetingModule = true;
librarycategory.id = 1;
projectsegmentcategory.librarycategory = librarycategory;
def projectsegmentcategoryvalues_list = [projectsegmentcategoryvalue]
projectsegmentcategory.projectsegmentcategoryvalues = new TreeSet<ProjectSegmentCategoryValue>();
projectsegmentcategory.projectsegmentcategoryvalues.add(Mock(ProjetSegmentCategoryValue))
projectsegmentcategory.projectsegmentcategoryvalues.first().id = 1
projectsegmentcategoryvalue.isClustered = true
projectsegmentcategoryvalue.isUnlockedForBudgetingModule = true
librarycategoryvalue.id = 1
projectsegmentcategoryvalue.projectsegmentcategory = projectsegmentcategory
projectsegmentcategoryvalue.librarycategoryvalue = librarycategoryvalue
projectsegmentclassification.id = 1
projectsegmentcategory.projectsegmentclassification = projectsegmentclassification
}
void "Test ProjectSegmentCategory Gson with default values"() {
when:"A ProjectSegmentCategory gson is rendered"
def renderResult = render(template: "/project/instance/projectSegmentCategory/projectSegmentCategory", model:[projectsegmentcategory: projectsegmentcategory])
then:"The json is correct"
// Mock the method getName as the real method is more complicate and requires a Language object
2 * projectsegmentcategory.getName(_ as String) >> "Test"
2 * projectsegmentcategory.getName() >> "Test"
2 * projectsegmentcategory.toString() >> "Test"
2 * projectsegmentcategoryvalue.getName(_ as String) >> "Test"
1 * projectsegmentcategoryvalue.getTranslationByLanguageId(_ as Long) >> "Test"
1 * projectsegmentcategoryvalue.getId() >> "1"
//1 * ProjectSegmentCategoryValue.getTranslationByLanguageId(_ as Long) >> "Test"
1 * projectsegmentcategory.projectsegmentcategoryvalues.iterator(_) >> projectsegmentcategoryvalue.iterator()
// Tests
renderResult.json.id == 1
renderResult.json.isClustered == true
renderResult.json.isUnlockedForBudgetingModule == true
renderResult.json.librarycategoryvalue.id == 1
renderResult.json.projectsegmentcategory.id == 1
renderResult.json.name == "Test"
}
}