qt.qml.typeregistration: Invalid QML element name "CalendarSystem"; value type names should begin with a lowercase letter

The combination of these things…

class CalendarSystem
{
    Q_GADGET

public:
    enum System : int {
        Gregorian = static_cast<int>(QCalendar::System::Gregorian),
        // ...
    };
    Q_ENUM(System)
};
Q_DECLARE_METATYPE(CalendarSystem)

class AlternateCalendarConfigPlugin : public QQmlExtensionPlugin
{
    Q_OBJECT
    Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QQmlExtensionInterface")

public:
    void registerTypes(const char *uri) override
    {
        qmlRegisterUncreatableType<CalendarSystem>(uri, 1, 0, "CalendarSystem", QStringLiteral("Error: only enums"));
    }
};

…results in the following error:

qt.qml.typeregistration: Invalid QML element name "CalendarSystem"; value type names should begin with a lowercase letter