How do I add a subclass to a List<superclass> in Realm Swift

I have a superclass called Thing (I know it's not specific) that inherits from Object:

class Thing: Object {
    dynamic var title: String = ""

    var parents: [Thing] {
        return linkingObjects(Thing.self, forProperty: "children")
    }

    let children = List<Thing>()
}

and two subclasses of that called Task and Thought.

However, when I do

someThing.children.append(Task())

it throws "Object type is incorrect."

What am I doing wrong? Does realm not allow subclasses in lists of superclasses because that doesn't make much sense.