(Sorry for the thread hijack!)
Thanks for checking Dwight,
It works fine with the default empty AC12 template. It seems to fail for me when using a particular file originated in AC11, which I have based my blank AC12 file on, even if it is empty of geometry.
Perhaps AC12 doesn't like using AC11 attributes somehow, or has become corrupted?
I suspect the problem is that I don't use a proper 'template' as such, but I strip down a copy of the previous file and use that as the basis of the new one, as I keep tweeking and adding stuff to it. Maybe it is time I learnt how to set up and manage a proper template!
For the time being I will create a fresh AC12 template from scratch, import in any custom attributes from another file, and see if that sorts it.