PNG transparency in PGD 9 vs 2013

[Sim]

I have an image that I need to export as a PNG with a transparent background.

In PGD 2013 it works fine - PNG is saved with a transparent background.

However, in PGD 9, it saves with a white (non-transparent) background.

There was a warning dialog about transparency pop up the first time I used PGD 9 to export the PNG, but I can't remember exactly what it said - something about transparency not supported for this type of file I think. I chose to ignore always and I can't find anywhere I can re-enable that option to see if it fixes things.

Any suggestions?

I've attached the xar file.