I'm using the Jira REST API (specifically Node.js jira-connector library) to aggregate a specific field's content from each of the issue's subtasks and using the aggregated result to populate a field in the parent issue. Once I've built the string I'd like to update the parent ticket with, I call the jira.issue.editIssue function and get the following error:

The screen for viewing, editing and creating issues in my project are all configured to have the specified custom field, and the custom field appears when I query the issue with the current value. This is all being done in a Jira Software project.
I'm not sure if this is a project configuration, screen / issue configuration or a custom field configuration problem. I'm fairly new to working with the Jira API so any help is appreciated!