System.Exception: " не является допустимым значением для 'Double'."

public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            cbFF.ItemsSource = Fonts.SystemFontFamilies.OrderBy(f => f.Source);
            cbFS.ItemsSource = new List<double>() { 8, 9, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 48, 72 };
        }
 
        private void Open_Executed(object sender, ExecutedRoutedEventArgs e)
        {
            OpenFileDialog dlg = new OpenFileDialog();
            dlg.Filter = "Rich Text Format (*.rtf)|*.rtf|All files (*.*)|*.*";
            if (dlg.ShowDialog() == true)
            {
                FileStream fileStream = new FileStream(dlg.FileName,FileMode.Open);
                TextRange range = new TextRange(rtb.Document.ContentStart, rtb.Document.ContentEnd);
                range.Load(fileStream, DataFormats.Rtf);
            }
        }
 
        private void Save_Executed(object sender, ExecutedRoutedEventArgs e)
        {
            SaveFileDialog dlg = new SaveFileDialog();
            dlg.Filter = "Rich Text Format (*.rtf)|*.rtf|All files (*.*)|*.*";
            if (dlg.ShowDialog() == true)
            {
                FileStream fileStream = new FileStream(dlg.FileName, FileMode.Create);
                TextRange range = new TextRange(rtb.Document.ContentStart, rtb.Document.ContentEnd);
                range.Save(fileStream, DataFormats.Rtf);
            }
        }
 
        private void cbFF_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            if (cbFF.SelectedItem != null)
                rtb.Selection.ApplyPropertyValue(Inline.FontFamilyProperty, cbFF.SelectedItem);
        }
 
        private void cbFS_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            rtb.Selection.ApplyPropertyValue(Inline.FontSizeProperty, cbFS.Text);
        }
 
        private void rtb_SelectionChanged(object sender, RoutedEventArgs e)
        {
            object temp = rtb.Selection.GetPropertyValue(Inline.FontWeightProperty);
            btBold.IsChecked = (temp != DependencyProperty.UnsetValue) && (temp.Equals(FontWeights.Bold));
            temp = rtb.Selection.GetPropertyValue(Inline.FontStyleProperty);
            btIt.IsChecked = (temp != DependencyProperty.UnsetValue) && (temp.Equals(FontStyles.Italic));
            temp = rtb.Selection.GetPropertyValue(Inline.TextDecorationsProperty);
            btul.IsChecked = (temp != DependencyProperty.UnsetValue) && (temp.Equals(TextDecorations.Underline));
            temp = rtb.Selection.GetPropertyValue(Inline.FontFamilyProperty);
            cbFF.SelectedItem = temp;
            temp = rtb.Selection.GetPropertyValue(Inline.FontSizeProperty);
            cbFS.SelectedItem = temp.ToString();
        }
    }
}