When I use Multibinding, Converter not working collect

public partial class TestControl : UserControl
{
     public TestClass TimeData{get;set;}
     public TestControl ()
     {
        InitializeComponent();
     }
}
public class TestClass
{
        public int DelayTime { get; set; } = 80;
}

When I use this code,textblock.Text will be 80_80.

<Grid DataContext="{Binding TimeData, ElementName=userControl, Mode=OneWay}">
     <TextBlock Grid.Row="1">
          <TextBlock.Text>
               <MultiBinding StringFormat="{}{0}_{1}">
                     <Binding Path="DelayTime"/>
                     <Binding Path="DelayTime"/>
               </MultiBinding>
          </TextBlock.Text>
     </TextBlock>
</Grid>

But When i add Converter, It is not working
object[] values is 0,0;

<Grid DataContext="{Binding TimeData, ElementName=userControl, Mode=OneWay}">
     <TextBlock Grid.Row="1">
          <TextBlock.Text>
               <MultiBinding Converter="{StaticResource MultiflyConverter}">
                     <Binding Path="DelayTime"/>
                     <Binding Path="DelayTime"/>
               </MultiBinding>
          </TextBlock.Text>
     </TextBlock>
</Grid>

public class MultiflyConverter : IMultiValueConverter
{
     public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
        {
            var a = System.Convert.ToInt32(values[0]);
            var b = System.Convert.ToInt32(values[1]);
            return a * b;
        }
     public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }

How can i fix it?