Problems Packaging AIR Native Package: "Unable to build a valid certificate chain for the signer"

I'm using IntelliJ-IDEA 2018.1 to create a Desktop AIR app, the app works great running and debugging from IntelliJ. But trying to package it as a native installer (DMG), it won't sign. I keep getting this error:

"Unable to build a valid certificate chain for the signer."

I generated a 3rd Party Mac Developer Application certificate on the Apple Developer site, installed it into my keychain, and exported it and the private key to a p12, and provided that in IntelliJ module AIR Package -> Keystore file and unchecked "Use self-signed certificate", then chose Build -> Package AIR Application and chose native installer because I am packaging an ANE and you can't do that if you package an AIR file. It has to be a native package.

 

I've tried selecting the 3rd Party Mac Developer Application certificate as well as the Apple Worldwide Developer Relations Certification Authority and the Developer ID Certification Authority in Keychain Access app and exported the combined certificates as p12 to hopefully provide the entire "chain" for signing, but that fails with the same error.

I've generated a 3rd Party Mac Developer Installer certificate, and tried using that but I get a different error in that case:

"Failed to package AIR application desktop-sandbox.dmg: not an X509 code-signing certificate"

My ADT command line in this case:

/Applications/IntelliJ IDEA.app/Contents/jdk/Contents/Home/jre/bin/java -Dapplication.home=/Users/ethomas/linqto/FlexSDKs/SDK4.16.1-29a -Dfile.encoding=UTF-8 -Djava.awt.headless=true -Duser.language=en -Duser.region=en -Xmx1024m -jar /Users/ethomas/linqto/FlexSDKs/SDK4.16.1-29a/lib/adt.jar -package -storetype PKCS12 -keystore /Users/ethomas/linqto/FlexDev/certs/MacOSInstallerCert.p12 -target native /Users/ethomas/linqto/FlexDev/navcon/out/production/desktop-sandbox/desktop-sandbox.dmg /Users/ethomas/linqto/FlexDev/navcon/out/production/desktop-sandbox/desktop-sandbox-app.xml -extdir /Users/ethomas/linqto/FlexDev/EventANE -C /Users/ethomas/linqto/FlexDev/navcon/out/production/desktop-sandbox Main.swf

 

Do you have any ideas what I'm doing wrong?

 

Thanks very much for your help.