Why can't I bind the BeginTime property of this animation?

Question

So I have a control template that looks a bit like this

            <ControlTemplate TargetType="{x:Type local:OptionCheckBox}">
                <Grid>...</Grid>
                <ControlTemplate.Triggers>
                    <Trigger Property="Visibility" Value="Visible">
                        <Trigger.EnterActions>
                            <BeginStoryboard>
                                <Storyboard>
                                    <DoubleAnimation Storyboard.TargetProperty="Opacity" From="0" To="1" Duration="0:0:0.5" BeginTime="{TemplateBinding AnimationDelay}"  />
                                </Storyboard>
                            </BeginStoryboard>
                        </Trigger.EnterActions>
                    </Trigger>
                </ControlTemplate.Triggers>
             </ControlTemplate>

This is a default template for a control that defines the dependency property:

    public static readonly DependencyProperty AnimationDelayProperty;

    static OptionCheckBox()
    {
        var animationDelayMeta = new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.None, new PropertyChangedCallback(AnimationDelayChangedHandler));

        AnimationDelayProperty = DependencyProperty.Register("AnimationDelay", typeof(TimeSpan?),
                                                             typeof(OptionCheckBox), animationDelayMeta);
    }

Everything looks perfectly legit to me, yet when I run this I get the error

Cannot convert the value in attribute 'BeginTime' to object of type ''.

So as the BeginTime property type is Nullable I thought this would be fine. I have tried setting the default value just to new TimeSpan(), but this doesn't work. I'm sure there's something glaringly obvious. Any ideas?