Json issue in a sub-class

Question

I have 2 DTO files, with one of them being a sub-class of the other. The fields on the super-class are as follows:

[JsonProperty]
protected int financialFormatHeaderID;

[JsonProperty]
protected string financialFormatHeaderCode;

[JsonProperty]
protected string description = string.Empty;

[JsonProperty]
protected FinancialFormatPurpose purpose = FinancialFormatPurpose.Standard;

[JsonProperty]
protected IList<FinancialFormatDetailDto> details = new List<FinancialFormatDetailDto>();

[JsonProperty]
protected bool active = true;

The sub-class has this:

public RecoveryFormatHeaderDto()
  : base() {
  this.purpose = FinancialFormatPurpose.Recovery;
}

[JsonProperty]
private bool isPerSquareArea;

They both have the JsonObject(MemberSerialization.OptIn) and Serializable attributes above the class name (but under the namespace).

My problem is that when I try to save an instance of the sub-class I get the following error:

Newtonsoft.Json.JsonSerializationException was unhandled by user code

Message=A member with the name 'financialFormatHeaderID' already exists on 'RecoveryFormatHeaderDto'. Use the JsonPropertyAttribute to specify another name.

Source=Newtonsoft.Json

What is happening here? How do I fix this?

EDIT: Class signatures are as follows:

public class FinancialFormatHeaderDto
public class RecoveryFormatHeaderDto : FinancialFormatHeaderDto