update.Value() does not work in Post Function

[Former user]

Attached is a script I can run in Code Runner groovy console and everything works fine. When I try running it as a post function it fails on the last step which is the update.Value. The only difference between this script and the post function script I use is the post function script does not have the two lines of code at the top to retrieve an issue and I use the binding variable $issue. So issue is replaced with $issue in the post function.

I'm using Jira 8.6.0 server