WPF DataContext in ToolTip

Question

I'm trying to bind the tooltip of a ListBoxItem. I have this defined in my ControlTemplate:

<ControlTemplate TargetType="{x:Type ListBoxItem}">
    <ControlTemplate.Resources>
        <conv:IconConverter x:Key="IconConverter" />
        <conv:FirstImageConverter x:Key="FirstImageConverter"/>
        <conv:DebugConverter x:Key="dbg"/>
    </ControlTemplate.Resources>
<Border ...>
    <StackPanel>
        <Image MaxHeight="160" Stretch="UniformToFill" Source="{Binding Icon,Converter={StaticResource IconConverter},ConverterParameter=128}" Height="128"/>
        <TextBlock x:Name="lblName" Text="{Binding Name}" />
    </StackPanel>
    <Border.ToolTip>
        <DockPanel LastChildFill="True" Margin="2" MaxWidth="800" 
        DataContext="{Binding Images,Converter={StaticResource FirstImageConverter}}" 
        IsEnabled="{Binding Images,Converter={StaticResource FirstImageConverter},ConverterParameter=enabled}">
            <TextBlock DockPanel.Dock="Bottom" Width="Auto" MaxWidth="600"  Text="{Binding Caption}" Height="Auto" />
            <Image x:Name="imgFullSize" DockPanel.Dock="Top" Stretch="None" Width="Auto" Source="{Binding Filename,Converter={StaticResource IconConverter}}"/>
        </DockPanel>
    </Border.ToolTip>

Yes I get a binding error: System.Windows.Data Error: 3 : Cannot find element that provides DataContext. BindingExpression:Path=Images; DataItem=null; target element is 'DockPanel' (Name=''); target property is 'DataContext' (type 'Object')

My ListBoxItem's datacontext does contain the images property as it does the name and icon properties, which are displayed correctly. I've also tried using TooltipService.ToolTip instead of Border.ToolTip, with no effect. What's the difference, and why isn't my binding working?

Answer 1

Eventually solved this by changing the tooltip to this:

...
<Border.ToolTip>
    <ToolTip DataContext="{Binding PlacementTarget.DataContext.Images, RelativeSource={RelativeSource Self}}" Visibility="{Binding PlacementTarget.DataContext.Images, RelativeSource={RelativeSource Self},Converter={StaticResource FirstImageConverter},ConverterParameter=visible}">
        <DockPanel LastChildFill="True" Margin="2" MaxWidth="800">
            ...

Thanks @Rachel for leading me to the answer.