Unhandled Exception: InvalidCastException

[ghost]

Stack trace:
System.InvalidCastException: Specified cast is not valid.
at Patchy.Converters.NetworkSpeedConverter.ConvertBack(Object value, Type targetType, Object parameter, CultureInfo culture)
at System.Windows.Data.BindingExpression.ConvertProposedValue(Object value)
at System.Windows.Data.BindingExpressionBase.UpdateValue()
at System.Windows.Data.BindingExpression.UpdateOverride()
at System.Windows.Data.BindingExpressionBase.Update()
at System.Windows.Data.BindingExpression.OnLostFocus(Object sender, RoutedEventArgs e)
at System.Windows.WeakEventManager.ListenerList`1.DeliverEvent(Object sender, EventArgs e, Type managerType)
at System.Windows.WeakEventManager.DeliverEvent(Object sender, EventArgs args)
at System.Windows.LostFocusEventManager.OnLostFocus(Object sender, RoutedEventArgs args)
at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
at System.Windows.UIElement.RaiseEvent(RoutedEventArgs e)
at System.Windows.Controls.Primitives.TextBoxBase.OnLostFocus(RoutedEventArgs e)
at System.Windows.UIElement.IsFocused_Changed(DependencyObject d, DependencyPropertyChangedEventArgs e)
at System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
at System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
at System.Windows.Controls.TextBox.OnPropertyChanged(DependencyPr