Undefined index error in tool_launch service when only 'lineitem' property is present

We're trying to integrate with a client that has Moodle acting as a Tool.

While sending the 'id_token', we only send a 'lineitem' property without the 'lineitems' (plural) property, since we only support sending scores to a single resource.

When launching we are getting the following error:

Notice: Undefined index: lineitems in /var/www/*******/public_html/enrol/lti/classes/local/ltiadvantage/service/tool_launch_service.php on line 166

Upon inspecting this piece of code, it seems to be getting the 'lineitem' only when the plural 'lineitems' is present:

        // AGS. If the lineitemsurl is missing, it means the tool has no access to the endpoint.

        // See: http://www.imsglobal.org/spec/lti-ags/v2p0#assignment-and-grade-service-claim.

        if ($launchdata->ags && $launchdata->ags['lineitems']) {

            $resourcelink->add_grade_service(

                new \moodle_url($launchdata->ags['lineitems']),

                isset($launchdata->ags['lineitem']) ? new \moodle_url($launchdata->ags['lineitem']) : null,

                $launchdata->ags['scope']

            );

        }

According to the spec, the plural 'lineitems' is optional.
What baffles me is that this piece of code has a comment with a link to the IMS spec, which literally has an example without the plural 'lineitems':
 

I assume this is a bug? This is preventing us from completing our integration with this client.