Flask: Flash function ignoring the 'category' argument

So I write the controller:

@app.route('/')
def index():
    flash('Hello world!', 'success')
    return render_template('index.html')

then in my template I output the flash messages like this:

{%- with messages = get_flashed_messages(with_categories=true) -%}
{%- if messages -%}
    <ul class="flashes unstyled">
    {%- for category, message in messages -%}
        <li class="alert alert-{{ category }}">
            <a class="close" data-dismiss="alert">&times;</a>
            {{ message }}
        </li>
    {%- endfor -%}
    </ul>
{%- endif -%}
{%- endwith %}

But the issue is that I ALWAYS get just 'message' category so <li> goes with classes 'alert alert-message'.
I read the docs and as to me I did everything right, but 'flash' function ignoring the second argument and always uses the default value 'message' (instead of given by me 'success').

I wonder if anyone had that issue and know how to handle it?

Solution

Edit: Based on other comments and testing the kwarg is unnecessary.

Based on the docs at http://flask.pocoo.org/docs/api/#message-flashing it appears you need to use this format. flash(message, category='message')

@app.route('/')
def index():
    flash('Hello world!', category='success')
    return render_template('index.html')