unit test - count() is not working on a class extending an abstract class (Domain Mocks)

class Group {
	String name
	static hasMany = [members:Member]
    static constraints = {
    }
	Collection members
}

import groovy.transform.ToString

import java.util.Map


@ToString (includeNames=true,includeFields=true,excludes="xmlProperties")
abstract class Member   {
	
	
	String name
	String externalId
}

@ToString (includeNames=true,includeFields=true,excludes="xmlProperties")
class SubMember extends Member {
	
	
	String extraName
}

@TestFor(Group)
@Mock([SubMember])
class GroupTests {
	Group group
	SubMember subMember
	
	@Before
	void setUp() {

		group = new Group(name:"my group")
		subMember = new SubMember(name:"my name",extraName:"extra name")
		group.addToMembers subMember
		group.save(failOnError:true)
	}
	@Test
	void testSampleStructure()
	{
		//check that the group is saved
		assertEquals 1, Group.count()
		//get the saved group
		Group savedGroup = Group.findById(group.id)
		//check its not null
		assertNotNull savedGroup
		//check that it has 1 member
		assertEquals 1, savedGroup.members.size()
		//somehow the count is always zero		
		assertEquals 1, SubMember.count()
		 
		
	}
	
}