Cannot export transparent PNG animation frames in cycles render Newbie here, working with Blender for less than a week, so forgive my ignorance!

I've a simple 100 frame animation I'm trying to export with a transparent background. 

Under "Output", I've picked PNG, with RGBA ticked with a color depth of 16. Under "Film", transparent is ticked.



I can render a single image (F12), and I end up with the correct transparency, but when I render it as an animation, the transparency is gone on all the frames, replaced with black. Check out the two images below - one is the animation frame export, one is the single image export. 

What am I doing wrong?
I did not have "use alpha" checked in my compositing node. Does that make sense...?