Why can't I merge a solid with a PNG (alpha) ?

I did a merge between solid (input node A) and a PNG (input node B) but this simple setup doesn’t work. What am I doing wrong?

The solid goes to B.
The PNG to A.

there is CRC error in the GIMP PNG.

Setup worked at the 2nd time


Benjamin Röhling

He’s using Merge under (which is rather unusual in a compositing graph), so A is under B