Dynamic feedback parameter error "specify value for choices parameter"

[SaoirseConnorDesai]

Hello!

I'm trying out the feedback function from the Dynamic parameters overview page from the jsPsych website.

I've very closely adapted the code from the page and get this error You must specify a value for the choices parameter in the html-button-response plugin.

This is my code:

var practice_2_1 = {
        type: 'html-button-response',
        choices: ['Yes', 'No'],
        margin_horizontal: '140px',
        post_trial_gap: 1000,
        stimulus: ["<p align = 'center'><b>Practice problem 1</b></p>" +
            "<p align = 'center'>Please indicate whether you endorse the statement.<br><br> " +
            "<table class='center'>" +
            "<tr><td><b>Tray A</b></td><td><b>Tray B</td></tr> " +
            "<tr><td><p align = 'center'><img src = 'RB-beans/0-10.svg' style='width:auto;max-height:100px;' alt = 'Example tray with 10 beans, all of them white'</p></td>" +
            "<td><p align = 'center'><img src = 'RB-beans/100-100.svg' style='width:auto;max-height:320px;' alt = 'Example tray with 100 beans, all of them red'</p></td></tr>" +
            "</table>" +
            "<p align = 'center'>Choosing Tray B is more likely to win you a red bean.<br>" +
            "<p align = 'center'>Is this correct?<br>"],
        data: {
            correct_response: jsPsych.timelineVariable('correct_response'),
            trial_part: 'practice_2_1'
        },
        on_finish: function (data) {
            // Score the response as correct or incorrect.
            if (jsPsych.pluginAPI.compareKeys(data.response, "Yes")) {
                data.correct = true;
            } else {
                data.correct = false;
            }
        }
    };

var feedback_2_1 = {
        type: 'html-button-response',
        stimulus: function () {
            // The feedback stimulus is a dynamic parameter because we can't know in advance whether
            // the stimulus should be 'correct' or 'incorrect'.
            // Instead, this function will check the accuracy of the last response and use that information to set
            // the stimulus value on each trial.
            var last_trial_correct = jsPsych.data.get().last(1).values()[0].correct;
            if (last_trial_correct) {
                return "<p>Correct!</p>"; // the parameter value has to be returned from the function
            } else {
                return "<p>Wrong.</p>"; // the parameter value has to be returned from the function
            }
        }
    }

Can anyone tell me why I'm getting this error? I thought I didn't have to specify the choices parameter for the feedback trial. Perhaps I've missed something else?