Expression Incomplete Error

[Zara]

I'm creating this static custom dimension which is straightforward but its throwing an error saying expression incomplete. can someone help?

CASE
WHEN ${email_messages.is_sent} THEN "Sent Emails"
WHEN ${email_messages.is_opened} THEN "Opened Emails"
WHEN ${email_messages.is_clicked} THEN "Clicked Emails"
ELSE "No Data"
END