Mongodb $pull operator not removing element even though callback error is "null"

Question

I am trying to remove an array element from a MongoDB collection in Node.js. The problem is, that although the callback returns 1 and its error is null which should indicate that the operation was successful, right? But when I check the database, the element I wanted to delete is still there. If I execute the same command in the Mongo Shell it works. I really don't see what I am doing wrong. Does anyone have an idea?

My object in the collection:

{
    "_id" : ObjectId("55dc82bd0ab9de6c20bc2eb9"),
    "title" : "post title",
    "content": "post content",
    "time" : 1440514750,
    "comments" : [
        {
            "time" : 1440683176,
            "username" : "arne",
            "content" : "This is my comment"
        },
        {
            "time" : 1440683198,
            "username" : "jack",
            "content" : "This is another comment"
        }
    ]
}

Working command in mongo shell (successfully removes comment from collection):

> db.posts.update({_id: ObjectId("55dc82bd0ab9de6c20bc2eb9")}, {$pull: {comments: {time: 1440683198}}})
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

Not working function in node.js:

req.db.collection('posts').update({_id: new mongoID(postId)}, {$pull: {comments: {time: commentTime}}}, function(err, result) {
    console.log('result', result);
    console.log('err', err);
    res.redirect('/blog/' + postId);
});

Output:

object 1
err null

but the comment I wanted to delete is still there