TypeConverter broken in SL2B2?

In SL2B1 I was able to use TypeConverter to convert strings in Xaml to actual types, but it isn't working in SL2B2. I have the following:
    [TypeConverter(typeof(ExtentConverter))]
    public struct Extent
    {
        public Extent(double minx, double miny, double maxx, double maxy)
        { /* stuff here */ }
    }
which uses the following TypeConverter:
    public class ExtentConverter : TypeConverter
    {
        public override object ConvertFrom(object value)
        {
            string strValue = value as string;
            if (strValue != null)
  &n ...