.GreaterThan filter not working on List count?

I'm trying to achieve filtering a result based on the Count-property of a List being greather than zero, like this:

query = query.Filter(u => u.SomeList.Count.GreaterThan(0));

This doesn't work, or rather it causes the search to return no hits. I've confirmed that there should be multiple valid results for this filter. 

Any idea what is wrong here?