FileHelpers FieldConverter is not called if the value is blank

I am trying to parse a file using FileHelpers. I need to map fields to KeyValuePair and there is a mapping for some of these fields if the line in the file is a space. However, my custom FieldConverter FieldToString method doesn't seem to be called when the line from the file is a space. I want it to be called though!

Here is my field definition:

[FieldFixedLength(1)]
[FieldTrim(TrimMode.Right)]
[FieldConverter(typeof(AreYouOneOfTheFollowingConverter))]
public KeyValuePair<int, string>? AreYouOneOfTheFollowing;

      

        
        
        
      

    

Here is my converter ([case "":] never gets hit):

public class AreYouOneOfTheFollowingConverter : ConverterBase
{
    public override object StringToField(string from)
    {
        switch (from)
        {
            case "1":
                {
                    return new KeyValuePair<int, string>(1469, "Yes");
                }
            case " ":
                {
                    return new KeyValuePair<int, string>(1470, "No");
                }
            default:
                {
                    if (String.IsNullOrWhiteSpace(from))
                    {
                        return from;
                    }
                    else
                    {
                        throw new NotImplementedException();
                    }
                }
        }
    }
}

      

        
        
        
      

    

Ideas?