Verification call always returns INVALID in google script

[ManiSubramaniyan]

General information

• Environment:
• PayPal-Debug-ID values:
• Language, language version, and OS:

Issue description

[ManiSubramaniyan]

We have a google domain with GSuite enterprise and so the IPN listener code is hosted there as javascript, based on the example from this git (only changes are done in the part after the transaction is verified and payment completed). I am getting successful IPN calls from Paypal but then when I do the verification call (UrlFetch call for verification), it always comes back with INVALID as answer.
Tried both ipn and paypal urls; tried both sandbox and production. Is the git sample for google script wrong or incomplete??
Thanks