MultiBinding fails Single Binding works

Hi

This works:

    public class answered : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            Double b = (Double)value;
            return b * 350;
        }

        // No need to implement converting back on a one-way binding 
        public object ConvertBack(object value, Type targetType,
            object parameter, System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    

+

<TextBlock Name="bestTimerRect" 
                               HorizontalAlignment="Left"
                               Background="Firebrick"
                               Width="{Binding proportionComplete, Converter={StaticResource answered}}"></TextBlock>

But, essentially the same operation, fails "Specified cast is not valid"  (I have simplified to one binding here)

    public class multiAnswered : IMultiValueConverter
    {
        public object Convert(object[] values, Type targetType,object parameter, System.Globalization.CultureInfo culture)
        {
            Double b = (Double)values[0];
            return b * 350;
        }
        public object[] ConvertBack(object value, Type[] targetTypes,
               object parameter, System.Globalization.CultureInfo culture)
        {
            throw new NotSupportedException("Cannot convert back");
        }
    }

+

<TextBlock Name="bestTimerRect" HorizontalAlignment="Left" Background="Firebrick"><TextBlock.Width><MultiBinding Converter="{StaticResource multiAnswered}"><Binding Path="proportionComplete"></Binding></MultiBinding></TextBlock.Width></TextBlock>

Any help appreciated

Z