Firebase startAt() not working

I'm trying to get data out of firebase with the startAt() but it always returns "null". My firebase setup is good because other methods do work.

This is the code that always returns null:

    firebase.database().ref('posts').startAt('-KkFbse7MCiJF0P-hjgI').once('value')
    .then(function (snapshot) {
        console.log(snapshot.val());
        res.send(snapshot);
    })
    .catch(function (error) {
        console.log(error);
    });

and this is how my database looks: Firebase database

via tomzinho