diff --git a/api-ref/source/instances.inc b/api-ref/source/instances.inc
index 1d635fd537..2987cad632 100644
--- a/api-ref/source/instances.inc
+++ b/api-ref/source/instances.inc
@@ -246,6 +246,7 @@ Response Parameters
    - replicas.links: instance_replica_links
    - replicas.links.href: instance_replica_link_href
    - replicas.links.rel: instance_replica_link_rel
+   - replica_of: replica_of_optional
    - configuration: configuration1
    - configuration.id: configuration_id
    - configuration.name: configuration_name
diff --git a/api-ref/source/parameters.yaml b/api-ref/source/parameters.yaml
index df3440d1f6..fb07c8ce1e 100755
--- a/api-ref/source/parameters.yaml
+++ b/api-ref/source/parameters.yaml
@@ -709,6 +709,12 @@ replica_of:
   in: body
   required: true
   type: string
+replica_of_optional:
+  description: |
+    The primary instance ID of this replica.
+  in: body
+  required: false
+  type: string
 restore_point:
   description: |
     The ``restorePoint`` object. Use this paramter