How do you enable reference tracking in WCF?

I'm getting a strange error from a WCF service. The service returns a CampaignEmailDTO object, which has a 1 to many relationship with CampaignEmailRuleDTO. How do I fix this error?

The error is:

Object graph for type 'xx.xx.CampaignEmailRuleDTO' contains cycles and cannot be serialized if reference tracking is disabled.




The classes are:

[Serializable]
Class CampaignEmailDTO {
        public int Id {...}
        public string Name {...}
        public string FromAddress {...}
        ......etc.....
}

[Serializable]
Class CampaignEmailRuleDTO {
        public CampaignEmailDTO() {...}
        public int Id {...}
        public string Name {...}
        public List<CampaignEmailRuleDTO> CampaignEmailRules {...}
        public List<CampaignEmailSendLogDTO> CampaignEmailSendLogs {...}
        ......etc.....
}