can't get the key of a datasnapshot

I am trying to get the key of a snapshot with the

dataSnapshot.key()

method, but it doesn't seem to be working. Here is the related code:

index.html:

...

<select id="resList" size="20"></select>

...

index.js:

function addChild(name, id) {
    var list = document.getElementById("resList");
    var item = document.createElement("option");
    item.text = "Resolution " + id + ": " + name;
    list.add(item);
}

function changeChild(name, index) {
    var list = document.getElementById("resList");
    var item = document.createElement("option");
    item.text = "Resolution " + (index+1) + ": " + name;

    list.remove(index);
    list.add(item, index);
}

function removeChild(index) {
    var list = document.getElementById("resList");
    list.remove(index);
}

function init() {
    const resolutionRef = firebase.database().ref().child('resolutions');

    resolutionRef.on('child_added', function(childSnapshot, prevChildKey) {
        if (prevChildKey == null)
            prevChildKey = "0";

        addChild(childSnapshot.val(), parseInt(prevChildKey) + 1);
    });

    resolutionRef.on('child_changed', function(childSnapshot, prevChildKey) {
        if (prevChildKey == null)
            prevChildKey = "0";

        changeChild(childSnapshot.val(), parseInt(prevChildKey));
    });

    resolutionRef.on('child_removed', function(oldChildSnapshot) {
        removeChild(parseInt(oldChildSnapshot.key()));
    });
}

window.onload = init;

I have a child of the 'root' reference called 'resolutions'. For each resolution a new child to the 'resolutions' reference is created. Each resolution has a name and an ID. I'm storing the ID of each resolution as the key and it's name as the value. This is convenient because this way I can determine the index of a resolution in 'resList' by simply subtracting one of it's key.

The above code works fine for adding and changing children, but for some reason when I remove a child nothing happens.

Thanks for any help in advance!