MultiBinding Problem

I have Rectangle that I want to flip in the X axis, I'm using ScaleTransform.
The problem is that i want to use converter for the multi binding, but when i use multi binding the Rectangle doesn't flip.
If i use single binding it works.

In the XAML just replay between the binding and the multi binding to see the result.

<Grid >
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<DockPanel >
<StackPanel Orientation="Horizontal" DockPanel.Dock="Left">
<TextBlock Text="FlipX"/>
<TextBox x:Name="FlipX" Text="1" Margin="0,0,5,0"/>
</StackPanel>
</DockPanel>
<Grid Grid.Row="1">
<Grid>
<Grid Width="100" Height="100" RenderTransformOrigin="0.5, 0.5">
<Grid.RenderTransform>
<TransformGroup>
<ScaleTransform>
<ScaleTransform.ScaleX>
<Binding ElementName="FlipX" Path="Text"/>
<!--<MultiBinding Converter="{StaticResource eibiConverter}">
<Binding ElementName="FlipX" Path="Text"/>
</MultiBinding>-->
</ScaleTransform.ScaleX>
</ScaleTransform>
</TransformGroup>
</Grid.RenderTransform>
<Rectangle x:Name="MyRect" Fill="LightBlue" >
</Rectangle>
<TextBlock Text="(0,0)" VerticalAlignment="Top" HorizontalAlignment="Left"/>
</Grid>
</Grid>
<Border Width="100" Height="100" BorderBrush="Black" BorderThickness="2"/>
</Grid>
</Grid>

The converter for the multi binding is:
public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
return -1;
}