diff --git a/core/lib/Drupal/Core/Config/Schema/Sequence.php b/core/lib/Drupal/Core/Config/Schema/Sequence.php
index 43d3134..8ad81a7 100644
--- a/core/lib/Drupal/Core/Config/Schema/Sequence.php
+++ b/core/lib/Drupal/Core/Config/Schema/Sequence.php
@@ -70,6 +70,7 @@ public function get($index) {
    */
   public function set($index, $value) {
     $this->offsetSet($index, $value);
+    return $this;
   }
 
 }
diff --git a/core/lib/Drupal/Core/TypedData/ListInterface.php b/core/lib/Drupal/Core/TypedData/ListInterface.php
index 6c6e5c3..3f8972b 100644
--- a/core/lib/Drupal/Core/TypedData/ListInterface.php
+++ b/core/lib/Drupal/Core/TypedData/ListInterface.php
@@ -64,7 +64,7 @@ public function get($index);
    * @param mixed
    *   Item to be stored at the specified position.
    *
-   * @return self
+   * @return static
    *   Returns the list.
    */
   public function set($index, $item);
diff --git a/core/lib/Drupal/Core/TypedData/Plugin/DataType/ItemList.php b/core/lib/Drupal/Core/TypedData/Plugin/DataType/ItemList.php
index 8ee50fe..415ada8 100644
--- a/core/lib/Drupal/Core/TypedData/Plugin/DataType/ItemList.php
+++ b/core/lib/Drupal/Core/TypedData/Plugin/DataType/ItemList.php
@@ -125,6 +125,7 @@ public function set($index, $item) {
         $item = $item->getValue();
       }
       $this->get($index)->setValue($item);
+      return $this;
     }
     else {
       throw new \InvalidArgumentException('Unable to set a value with a non-numeric delta in a list.');