Problems creating a zip update that works in stock recovery (built from AOSP)

Question

I am trying to create a zip update that works in stock (AOSP) on Android on a Nexus 4.

I am creating a zip update containing an updater script (in Edify) and signing it all with sinapk.jar and the platform keys that were generated with my AOSP build.

However, the zip is rejected when I try to deploy it via adb sideload.

Verifying update package... Installation aborted 

What am I going to do wrong?