ClientIdError at /openid/authorize - No exception message supplied

[stodge]

After I login I get this error:

ClientIdError at /openid/authorize
No exception message supplied

The POST to /openid/authorize includes the "next" parameter:

/openid/authorize?response_type=code&scope=openid+email&client_id=517813&redirect_uri=http://192.168.1.30:12000/oidc/callback/&state=

But when it tries to get the client from the DB, it doesn't have an ID to retrieve:

kwargs | {'client_id': ''}

My login form is this, which I think is correct:

    <form class="pure-form pure-form-stacked" method="POST" action="{% url 'oidc_provider:authorize' %}">
        {% csrf_token %}
        <fieldset>
            <legend>Please login</legend>
            <input type="hidden" name="next" value="{{ next }}">
            <input id="id_username" name="username" type="text" placeholder="Username" />
            <input id="id_password" name="password" type="password" placeholder="Password" />
            <label for="default-remember">
            <button type="submit">Sign In</button>
        </fieldset>
    </form>

So why isn't getting the parameters it needs?

Am I right in thinking that the flow is:

POST /openid/authorize
GET /accounts/login
POST /openid/authorize
(redirect back to client?)