XAML TemplateBinding to a Dependency Property

I have created my own User Control which is based on Button. Inside myButton I want a small icon depicting the type of myButton. If I hardcode the image source in the button template everything works great. But I would like to programmatically/declaratively show different icons inside myButton based on a dependency property which I have created zIcon. The XAML template code which should take care of this is simply:

    <Image Source="{TemplateBinding zIcon}"/>

However, when I run this code I get an immediate error:

    "The Given key was not present in the dictionary"

I assume it is talking about my dependency property zIcon somehow not being in the dictionary? As you can see in the code below, that TemplateBinding technique works great for setting the Content but doesn't work for a custom dependency property. Any suggestions about how to remedy this?

P.S. I have not included the dependency property class/code snippet. Suffice it to say, zIcon shows up just fine in the Visual Studio Properties window for myButton.

<UserControl.Resources>
        <ControlTemplate x:Key="thisTemplate" TargetType="{x:Type Button}">
            <Button x:Name="thisButton" />
            <ControlTemplate.Triggers>
                    <!-- Only a native Button type has IsPressed, myButton does not-->
                <Trigger Property="IsPressed" Value="True">
                    <Setter TargetName="thisButton" Property="Background" Value="LightGray" />
                </Trigger>
            </ControlTemplate.Triggers>
        </ControlTemplate>

        <Style TargetType="{x:Type uCtrl:myButton}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type uCtrl:myButton}">
                        <Button x:Name="thisButton" Width="100" Height="30" 
                                    BorderBrush="Transparent" Background="Transparent">
                            <Button.Content>
                                <StackPanel Orientation="Horizontal" HorizontalAlignment="Left">
                                    <Image Source="{TemplateBinding zIcon}"/>
                                    <ContentPresenter VerticalAlignment="Center" Content="{TemplateBinding Content}" />
                                </StackPanel>
                            </Button.Content>
                        </Button>
                        <ControlTemplate.Triggers>
                            <!-- This trigger will not work if I place it in the ControlTemplate definition -->
                            <Trigger Property="IsMouseOver" Value="True">
                                <Setter TargetName="thisButton" Property="Background" Value="LightCyan" />
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </UserControl.Resources>
    
    <Grid>
        <Button x:Name="btnPageType" Template="{StaticResource thisTemplate}" />
    </Grid>
</UserControl>

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