Cannot Make Binding to ZIndex from Another Element in WPF

I want to make binding to the ZIndex of another element in WPF in the same .xaml file but it does not work.

The element to be bound.

<Border 
    x:Name="BubbleTop" 
    CornerRadius="5" 
    Background="#EBF5EB" 
    Padding="8,6" 
    BorderBrush="LightGray" 
    BorderThickness="1" 
    Grid.ZIndex="3">
        <ContentPresenter />
</Border>

The element who initial a binding.

<TextBlock
    x:Name="statusText"
    Margin="..."
    Foreground="{Binding ElementName=BubbleTop, Path=Grid.ZIndex, Converter={StaticResource ToggleColorConverter}}"
    FontWeight="Bold"
    Text="..."/>

In the converter, it is set to change the Foreground color according to the ZIndex of the Border element.

public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
    int z = (int)value;
    if (z == 3)
        return "Red";
    else
        return "Blue";
}

But it does not work. Any hint?