setFormValue does not work on the Labels custom field

import static com.atlassian.jira.issue.IssueFieldConstants.*

def labels = getFieldById(LABELS)
labels.setFormValue(["my label", "another label"])

def label2 = getFieldByName("Labels Field")

log.warn "label2: $label2"
label2.setFormValue("test")
label2.setFieldOptions(["test"])