Unable to deserialize XML post to WCF service. What am I missing?

I have this class which is an input to my WCF service:

[DataContract]
public class LoginDTO
{
    [DataMember]
    public string Username { get; set; }

    [DataMember]
    public string Password { get; set; }
}

And here is the method in my service interface (IUserServices):

    [OperationContract]
    [FaultContract(typeof(ServiceException))]
    [ServiceKnownType(typeof(LoginDTO))]
    [WebInvoke(UriTemplate = "/login/", Method = "POST")]
    UserDTO LogIn(LoginDTO loginInfo);

Testing my service (using Postman) causes this error:

System.Runtime.Serialization.SerializationException: Unable to deserialize XML body with root name 'LoginDTO' and root namespace '' (for operation 'LogIn' and contract ('IUserServices', 'http://tempuri.org/')) using DataContractSerializer. Ensure that the type corresponding to the XML is added to the known types collection of the service.

And I post this: (Content-Type => application/xml)

<LoginDTO>
    <Username>MyUsername</Username>
    <Password>MyPassword</Password>
</LoginDTO>

What am I missing, or what am I doing wrong?