Could not be converted to a type library - Error: Element not found

_{NOTE: I am answering my own question in the event it helps others in the future.}

I'm getting the error:

The assembly "C:\XYZ.dll" could not be converted to a type library. Type library exporter encountered an error while processing 'XYZ'. Error: Element not found.

Here is the code that causes the problem:

[Guid("7a4e9867-96a7-43f0-9492-0327b9053853"),
ClassInterface(ClassInterfaceType.None)]
public class TimeSeriesPoint
{
    public string Date { get; set; }
    public float Value { get; set; }
}

[Guid("7a4e9867-96a7-43f0-9492-0327b9053853"),
InterfaceType(ComInterfaceType.InterfaceIsDual)]
public interface IDataHelper
{
    //RCOMServerLib.IStatConnector Connector { set; }
    string Text { set; }
    void DoCallback();