WM3_MC_BuildFromNode not working with button press?

Hey folks, I've got a pretty simple piece of code that doesn't seem to be behaving as expected at all. After a couple of hours of head scratching I thought I'd seek help.

Here's the script:

 on bigBtn pressed do
 (
 originalMesh = $originalBox
 newMesh = $newBox
 originalMeshCheck = copy originalMesh
 compatibilityCheck = (Morpher())
 addModifier newMesh compatibilityCheck
 WM3_MC_BuildFromNode compatibilityCheck 1 originalMeshCheck
 --Then there'd be an if statement to check if morph data present
 )

I've put the first two variables as hardcoded, but they are grabbed via a pick button.

So it's a pretty simple script, but at the end of it I'm left with the newMesh having a morph target and no data.

What's SUPER confusing is that if I just run this without a button press, just that code on its own, it works flawlessly.