PropertyList not working - can not be mapped to a MetaDataType

I'm on EPiServer CMS 9.6.1 and have been following Per Magne Skuseth's blog post "Trying out PropertyList". I can see plenty of others commenting on that post who have gotten much further so it must be possible.. But for me when I have the property like this:

public virtual IList CustomerSkus { get; set; }

I keep getting the NotSupportedException with "The property data type IList can not be mapped to a MetaDataType".

And I've tried putting [BackingType(typeof(CustomerSkuListProperty))] and I then get NotSupportedException "The property data type Json can not be mapped to a MetaDataType".

I have definitely included the:

[PropertyDefinitionTypePlugIn]
public class CustomerSkuListProperty : PropertyListBase
{ }

And have copied the PropertyListBase exactly as in his post so cannot figure why I get those exceptions..

Oh and the CustomerSku class is pretty simple like this:

public CustomerSku
{
    public string AccountNumer { get; set; }
    public string Sku { get; set; }
}

Anything I might have missed?