Planner - Update task: UnknownError

Question

I'm trying to update a task in planner using beta graph api. My call is PATCH: https://graph.microsoft.com/beta/tasks/dBwDqUFHpkCetYiUluQlnJYACAzh

The json object looks like this:

{
    "bucketId": "Tbi8MJwWwEu4SUYdrqt235YAEkke",
    "title": "New Title"
}

And the response:

Status Code: 412
{
    "error": {
        "code": "UnknownError",
        "message": "",
        "innerError": {
            "request-id": "43132ef7-9a88-4030-b8ad-873d2dc04ff0",
            "date": "2017-02-21T11:03:59"
        }
    }
}

Now I know that status code 412 means Precondition Failed, and I've set the If-Match Request Header. Do you have any other suggestion?

I've also tried the code in https://graph.microsoft.io/en-us/graph-explorer so it is clear that I'm missing something.

Thank you!