Trying to submit iron-form polymer 2 - Cannot read property 'submit' of null

I'm not able to submit my iron-form. At this point I just need to see the content in the console, but when trying to submit the form i'm only getting error-msg: Uncaught TypeError: Cannot read property 'submit' of null. I've probably missed something obvious. I've been using the page: https://www.webcomponents.org/element/PolymerElements/iron-form

<iron-form id="sizeForm">
        <form method="post" action="">
            <paper-dropdown-menu label="Choose type" on-iron-select="_typeSelected">
                <paper-listbox slot="dropdown-content">
                    <paper-item value="Sneakers">Sneakers</paper-item>
                    <paper-item value="Shoes">Shoes</paper-item>
                    <paper-item value="T-shirts">T-shirts</paper-item>
                    <paper-item value="Jeans">Jeans</paper-item>
                </paper-listbox>
            </paper-dropdown-menu>
            <add-sneakers hidden$="{{hideSneakers}}"></add-sneakers>
            <paper-button onclick="{{_submitForm}}">Accept</paper-button>
            <div class="output"></div>
        </form>
    </iron-form>

<script>
    _submitForm() {
    document.getElementById('sizeForm').submit();
    }
</script>

Change

<paper-button onclick="{{_submitForm}}">Accept</paper-button>

to

<paper-button on-tap="_submitForm">Accept</paper-button>

or

<button onclick="_submitForm()">Accept</button>

Also, it seems the iron-form docs got it wrong: document.getElementById('sizeForm').submit() works with a button but not a paper-button. this.$.sizeForm.submit() works with either. (I'll explore this more, and may submit a pull request on it.)

See this pen for an example.