No apparent means of clearing value in Epic Link field in a behaviour

In a behaviour associated to the Epic Link field, none of the following pieces of code when put in a server side script are successful in removing the name of an epic from the Epic Link field:

• epicLink.setFormValue(null)
• epicLink.setFormValue([])
• epicLink.setFormValue("")

In the Chrome developer console, there is no issue link field in the runvalid JSON and the following error appears:

["[SR Behaviours]", "Failed to read epic name field from issue:  - perhaps it doesn't exist or you don't have permissions to see it"]

This is preceded by a failed (405 Method Not Allowed) HTTP request to <jira_base_url>/rest/api/2/issue/?fields=customfield_10005&_=1518023764149, with customfield_10005 being the id of the Epic Name field. Both of the above occur in spite of the Epic Name field not being referenced anywhere in the behaviour.