When I bind a custom attached property, the first code binding fails. Why?

Question

<!-- Code 1 -->
<DataTrigger Binding="{Binding (ap:PasswordBoxProperties.HasText), RelativeSource={RelativeSource Mode=TemplatedParent}}" Value="False">
    <Setter Property="Visibility" Value="Visible"/>
</DataTrigger>

<!-- Code 2 -->
<DataTrigger Binding="{Binding Path=(ap:PasswordBoxProperties.HasText), RelativeSource={RelativeSource Mode=TemplatedParent}}" Value="False">
    <Setter Property="Visibility" Value="Visible"/>
</DataTrigger>

The Code 1 only omits the Path.

I think omitting the Path is supported, but it doesn't work. I wonder why this writing won't work, is it because it can't find this additional property?