Adding more than one property to a GUI built with Qt Designer causes SystemError in PyQt5

So I just discovered something which I believe is a bug, but maybe I did something wrong so thought I should post it here.

I designed a GUI using Qt Designer and then loaded that into my interface class using loadUi(). All of that works without any problems. However, then, I added properties to my class: A single property doesn't cause any issues, but as soon as there is more than one, I receive a SystemError:

Traceback (most recent call last):
  File "test.py", line 23, in <module>
    GUI = MainWindow()
  File "test.py", line 9, in __init__
    loadUi('../gui/test.ui', self)
  File "..\Anaconda3\lib\site-packages\PyQt5\uic\__init__.py", line 226, in loadUi
    return DynamicUILoader(package).loadUi(uifile, baseinstance, resource_suffix)
  File "..\Anaconda3\lib\site-packages\PyQt5\uic\Loader\loader.py", line 72, in loadUi
    return self.parse(filename, resource_suffix, basedir)
  File "..\Anaconda3\lib\site-packages\PyQt5\uic\uiparser.py", line 1021, in parse
    actor(elem)
  File "..\Anaconda3\lib\site-packages\PyQt5\uic\uiparser.py", line 942, in createConnections
    QtCore.QMetaObject.connectSlotsByName(self.toplevelWidget)
SystemError: <built-in function connectSlotsByName> returned a result with an error set

The UI that I loaded in this case is a plain QDialog template, nothing else. You can see the contents of test.ui below:

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>Dialog</class>
 <widget class="QDialog" name="Dialog">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>400</width>
    <height>300</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>Dialog</string>
  </property>
 </widget>
 <resources/>
 <connections/>
</ui>

Here is the code that created the error:

import sys
from PyQt5.QtWidgets import QDialog, QApplication
from PyQt5.uic import loadUi


class MainWindow(QDialog):
    def __init__(self):
        super().__init__()
        loadUi('../gui/test.ui', self)
        self.test = 1

    @property
    def test2(self):
        return self.test

    @property
    def test3(self):
        return self.test


if __name__ == '__main__':
    app = QApplication(sys.argv)
    GUI = MainWindow()
    GUI.show()
    sys.exit(app.exec_())

As soon as you remove one of the properties or the loadUi() command, it works again. What is wrong?