Have problem creating class diagram Follow

I have a problem getting the diagram feature to work as expected.

Imagine a project with the following classes:

class Base(object): pass
class SubClass1(Base): pass
class SubClass2(Base): pass

I right-click on SubClass1, select Diagram | Show Diagram and select Python class diagram.

Everything fine so far.

Then I press space (Add class to diagram) and select SubClass2.

SubClass2 is added to the diagram but (and this is the problem):

There is no line on the diagram indicating that SubClass2 is a subclass of Base.