Can someone explain why I can't search on the "Ancestors&&String" property?

So I have a simple method that uses a "dummy"-interface to get the "Ancestors" name so the underlaying FInd API can target the correct property, like this ->

    public interface ITracable
    {
        string[] Ancestors { get; set; }
        string CreatedBy { get; set; }    
    }

    mySearchClient.Search<IContent>().Filter(x => ((ITraceable)x).Ancestors.Match("100"))...

But it refuses to return results based on the search on that property. The document in Find looks like this (excerpt) ->

"CreatedBy$$string": "andre","Ancestors$$string": ["102486","102485","102481","55","1"
    ],

But with the same "Dummy interface" method I can target the "CreatedBy" property and I get search results. How can I correctly target the "Ancestors&&string" property? 
Thanks for pointing out what I do wrong!