Binding classes in Avalonia using a MultiBinding

Question

I am able to bind a class (class name is 'current' in this case) on a Border like this:

<Border Classes.current="{Binding CurrentStep}">
                ...
</Border>

But cannot work out the correct syntax for using a multi binding. This is what I tried:

<Border>
                <Border.Classes.current>
                    <MultiBinding Converter="{StaticResource AreAllEqual}">
                        <Binding Path="." />
                        <Binding Path="CurrentStep" />
                    </MultiBinding>
                </Border.Classes.current>
...
</Border>

But I get the error

Unable to resolve suitable regular or attached property Classes.current on type Avalonia.Controls:Avalonia.Controls.Border

How can I bind a class in Avalonia using a MultiBinding?