The problem comes from the Write node setup.
Look at the 'Input Premult' option, and set it to either 'UnPreMultiplied' or 'Opaque'.
That's the first way of fixing your problem.
The other one is to just add a shuffle before the Write node.
Connect the A input and set the A value to 1
Then you can render it without caring about the 'Input Premult' option.