Missing required icons - iTunes Connect - Mac app

I keep getting this error when uploading my packaged binary to iTunes Connect.

Missing required icons - The application bundle does not contain an icon in ICNS format, containing both a 512x512 and a 512x512@2x image. For further assistance, see the Apple Human Interface Guidelines.

I've tried codesigning and building the package manually and also via Mac App Store Toolset. I've tried using iConvert Icons to build a custom .icns file.

The App gets an icon in Finder, it's only after it's been uploaded that I get the icon is missing.

Have anyone else come across this problem?