Excel VBA crashing in MacOS

I have a simple Excel VBA macro to analyse and manipulate some powerpoint presentations. My code keeps crashing Excel when I run it for some reason. I've narrowed it down to an issue with referring to specific slides. A simplified version of the code, which just opens up a new presentation and adds a couple of blank slides to it, looks like this:

Option Explicit

Sub test()

Dim myApp As PowerPoint.Application

Dim myPres As Presentation

Dim mySlide As Slide

Set myApp = CreateObject("PowerPoint.Application")

Set myPres = myApp.Presentations.Add

Set mySlide = myPres.Slides.Add(Index:=1, Layout:=ppLayoutTitleOnly)

Set mySlide = myPres.Slides.Add(Index:=2, Layout:=ppLayoutTitleOnly)

' this next line causes a complete crash of Excel

Set mySlide = myPres.Slides(1)

' other code would follow here

End Sub

I can't work out what's going on. The code runs fine on a PC, so I'm guessing it's something to do with using a Mac. I'm running MacOS High Sierra, and Excel 16.16.1 (and the Powerpoint 16.0 object library). I get the same problem in my more complex full code - Excel crashes every time I try to refer to a specific slide in a presentation. 

All help greatly appreciated