Failed to serialize Entity Framework class webapi

I was searching for this issues but haven't found what I need. I need to use Entity Framework model in Web API for getting XML requests and responses.

I have two classes with a one-to-many relationship:

[Serializable]
[DataContract(Name = "shepherd")]
[KnownTypeAttribute(typeof(Sheep))]
public partial class Shepherd
{
        public Shepherd()
        {
            this.Sheep = new HashSet<Sheep>();
        }

        [DataMember(Name = "shepherdId")]
        //[XmlElement("shepherdId")]
        public int Id { get; set; }
        [DataMember(Name = "name")]
        //[XmlElement("name")]
        public string Name { get; set; }
        [DataMember(Name = "isDeleted")]
        //[XmlElement("IsDeleted")]
        public Nullable<bool> IsDeleted { get; set; }
        [DataMember(Name = "sheeps")]
        //[XmlArray("sheeps"), XmlArrayItem("sheep")]
        public virtual ICollection<Sheep> Sheep { get; set; }
    }

and this is the second class:

[DataContract(Name = "sheep")]
[KnownType(typeof(Shepherd))]
public partial class Sheep
{
        [DataMember(Name = "id")]
        //[XmlElement("id")]
        public int Id { get; set; }
        [DataMember(Name = "colour")]
        // [XmlElement("colour")]
        public string Colour { get; set; }
        [DataMember(Name = "createdon")]
        //[XmlElement("CreatedOn")]
        public Nullable<System.DateTime> CreatedOn { get; set; }
        public int Shepherd { get; set; }
        [IgnoreDataMember]
        public virtual Shepherd Shepherd1 { get; set; }
}

and I always get this error:

The 'ObjectContent`1' type failed to serialize the response body for content type 'application/xml; charset=utf-8'.

System.InvalidOperationException

Type 'System.Data.Entity.DynamicProxies.Shepherd_EF797FA5FAF7ACF0AF313B6DFF83AF6E5F2EE1AD57BC457F9C10E73BA5DEEFE6' with data contract name 'Shepherd_EF797FA5FAF7ACF0AF313B6DFF83AF6E5F2EE1AD57BC457F9C10E73BA5DEEFE6:http://ift.tt/1gVfT6f' is not expected. Consider using a DataContractResolver or add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.

I need to get only XML not Json