EF Core Second level ThenInclude missworks

Assume having these models first:

Method that has one OriginalCode OriginalCode that has many Mutants Mutant that has many ParseSubTrees

Now when querying on Method I want the other being loaded. So I have the following:

Method targetMethod = dBContext.Methods
            .Include(me => me.OriginalCode)
                .ThenInclude(oc => oc.Mutants)
            .FirstOrDefault(me => me.Id == id);

and the next step is to include additionally the ParseSubTree. But the thing is that I can't access it. See the following Image:

the problem is "mu is a list instead of being an object reference"!

Where is my mistake!

TG.