Merge pull request #1199 from segmentio/segment-sv-patch-1

Add userId is required for audiences/computed traits

Author: markzegarelli

src/connections/destinations/catalog/customer-io/index.md

@@ -230,6 +230,9 @@ Page events will only be associated to a user if the user has been previously id
 
 You can send computed traits and audiences generated using [Segment Personas](/docs/personas) to this destination as a **user property**. To learn more about Personas, contact us for a [demo](https://segment.com/contact/demo).
 
-For user-property destinations, an [identify](/docs/connections/spec/identify/) call is sent to the destination for each user being added and removed. The property name is the snake_cased version of the audience name, with a true/false value to indicate membership. For example, when a user first completes an order in the last 30 days, Personas sends an Identify call with the property `order_completed_last_30days: true`. When the user no longer satisfies this condition (for example, it's been more than 30 days since their last order), Personas sets that value to `false`.
+For user-property destinations, an [identify](/docs/connections/spec/identify/) call is sent to the destination for each user being added and removed. The property name is the snake_cased version of the audience name, with a true/false value to indicate membership. For example, when a user first completes an order in the last 30 days, Personas sends an Identify call with the property `order_completed_last_30days: true`. When the user no longer satisfies this condition (for example, it's been more than 30 days since their last order), Personas sets that value to `false`. 
 
-When you first create an audience, Personas sends an Identify call for every user in that audience. Later audience syncs only send updates for users whose membership has changed since the last sync.
+> note""
+> Customer.io requires you to pass a `userId` value when you sync Audiences or Computed Traits.
+
+When you first create an audience, Personas sends an Identify call for every user in that audience. Later audience syncs only send updates for users whose membership has changed since the last sync.