Validation exception thrown on adding object and inherited object to a collection-Domain Mocks - Unit test

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

class Member {

    static constraints = {
    }
	String name
}

@ToString (includeNames=true,includeFields=true)
class SubMemberA extends Member{

    static constraints = {
    }
	String extraNameA
}

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


/**
 * See the API for {@link grails.test.mixin.domain.DomainClassUnitTestMixin} for usage instructions
 */
@TestFor(Group)
@Mock([Group,SubMemberA,Member])
class GroupTests {

	Group group
	
	@Before
	void setUp() {
		
		group = new Group(name:"my group")
		def subMemeberA = new SubMemberA(name:"my name A",extraNameA:"extra name A")
		def member = new Member(name:"member")
		
		group.addToMembers subMemeberA
		group.addToMembers member
		group.save(failOnError:true)
	}
	@Test
	void testStructure() {
		
		assertNotNull Group.findById(group.id)
		
	}
	
}