diff --git a/services/notifications/README.md b/services/notifications/README.md
index 02e1a15e78..9db344dae4 100644
--- a/services/notifications/README.md
+++ b/services/notifications/README.md
@@ -2,3 +2,13 @@ notifications-sharelatex
 ===============
 
 An API for managing user notifications in ShareLaTeX
+
+
+database indexes
+================
+
+For notification expiry to work, a ttl index on `notifications.expiresFrom` must be created:
+
+```javascript
+db.notifications.createIndex({expiresFrom: 1}, {expireAfterSeconds: (60*60*24*30)})
+```
diff --git a/services/notifications/app/coffee/Notifications.coffee b/services/notifications/app/coffee/Notifications.coffee
index 4238ae37e9..1f0e1b8237 100644
--- a/services/notifications/app/coffee/Notifications.coffee
+++ b/services/notifications/app/coffee/Notifications.coffee
@@ -29,6 +29,8 @@ module.exports =
 					templateKey: notification.templateKey
 					expires: notification.expires || false
 				# ttl index on `expiresFrom` field
+				# in Mongo, TTL indexes only work on date fields,
+				# and ignore the document when that field is missing
 				if doc.expires
 					doc.expiresFrom = new Date()
 				db.notifications.insert(doc, callback)