Update fail when modifying the ROM update file.

Hi, I have a device with custom recovery that can update ROM using update files, I got a working update file (zip contains system/ and META-INT/), When I'm unzipping it, add files to it and zip it again then flashing it, I get update failed, I even get update failed when I'm just unzipping and zipping without adding files or touching the content in the zip. Does anyone know why it might happen?

Thanks.

from xda-developers http://ift.tt/2dqozW2
via IFTTT