Assigning to PyQt5 QQmlListProperty gives type error

_antipattern_

Hi,

I defined a custom type in python that contains a list of points that is exposed to qml as a QQmlListProperty. When assigning to it however, I get an error:

TypeError: list element must be of type 'QPointF', not 'NoneType'

I already checked the types of the list elements in QML and they are correct.
This is a minimal sample that demonstrates the error:
Python:

#!/usr/bin/python

import sys

from PyQt5.QtCore import QPointF, pyqtProperty
from PyQt5.QtGui import QGuiApplication
from PyQt5.QtQml import QQmlApplicationEngine, QQmlListProperty, qmlRegisterType
from PyQt5.QtQuick import QQuickItem


class CustomType(QQuickItem):

    _points = []

    @pyqtProperty(QQmlListProperty)
    def points(self):
        return QQmlListProperty(QPointF, self, self._points)

    @points.setter
    def points(self, points):
        self._points = points
        print(self._points)


def main():
    qmlRegisterType(CustomType, 'Example', 1, 0, 'CustomType')

    application = QGuiApplication(sys.argv)
    engine = QQmlApplicationEngine('qmllistpropertydemonstration.qml')

    engine.rootObjects()[0].setPoints([QPointF(1, 2), QPointF(3, 4)])

    return application.exec()


if __name__ == '__main__':
    main()

QML:

import Example 1.0

ApplicationWindow {
	width: 640
	height: 480

	function setPoints(points) {
		customtype.points = points
	}

	CustomType {
		id: customtype
	}
}

For now I fixed this by assigning the points in python instead of QML but I'd like to get rid of that workaround.
What am I doing wrong?