diff --git a/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php b/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php
index 6920bbb..ea36a87 100644
--- a/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php
+++ b/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php
@@ -1077,7 +1077,7 @@ protected function mapToStorageRecord(ContentEntityInterface $entity, $table_nam
         // @todo Give field types more control over this behavior in
         //   https://drupal.org/node/2232427.
         if (!$definition->getMainPropertyName() && count($columns) == 1) {
-          $value = $entity->$field_name->isEmpty() ? array() : $entity->$field_name->first()->getValue();
+          $value = ($item = $entity->$field_name->first()) ? $item->getValue() : array();
         }
         else {
           $value = isset($entity->$field_name->$column_name) ? $entity->$field_name->$column_name : NULL;
diff --git a/core/lib/Drupal/Core/Field/BaseFieldDefinition.php b/core/lib/Drupal/Core/Field/BaseFieldDefinition.php
index c353be2..9645de2 100644
--- a/core/lib/Drupal/Core/Field/BaseFieldDefinition.php
+++ b/core/lib/Drupal/Core/Field/BaseFieldDefinition.php
@@ -465,10 +465,18 @@ public function getOptionsProvider($property_name, FieldableEntityInterface $ent
     // If the field item class implements the interface, proxy it through.
     if (is_subclass_of($this->getFieldItemClass(), '\Drupal\Core\TypedData\OptionsProviderInterface')) {
       $items = $entity->get($this->getName());
-      // If the field is empty, create an item so that we can use it as the
-      // "options provider".
-      // @todo we should clear it afterwards ?
-      return $items->first() ?: $items->appendItem();
+      // We need to return a field item to be used as the "options provider". If
+      // the current entity already has such an item, use that one.
+      if ($item = $items->first()) {
+        return $item;
+      }
+      // If not, create a new item that has the entity as the parent, but do not
+      // modify the actual entity by leaving it there.
+      else {
+        $item = $items->appendItem();
+        $items->removeItem($item->getName());
+        return $item;
+      }
     }
     // @todo: Allow setting custom options provider, see
     // https://www.drupal.org/node/2002138.
diff --git a/core/lib/Drupal/Core/Field/FieldItemList.php b/core/lib/Drupal/Core/Field/FieldItemList.php
index bf23cf4..8a903da 100644
--- a/core/lib/Drupal/Core/Field/FieldItemList.php
+++ b/core/lib/Drupal/Core/Field/FieldItemList.php
@@ -127,14 +127,6 @@ public function setValue($values, $notify = TRUE) {
       if (!is_array($values) || !is_numeric(current(array_keys($values)))) {
         $values = array(0 => $values);
       }
-
-      // Non-numeric keys are not supported.
-      array_walk($values, function ($value, $index) {
-        if (!is_numeric($index)) {
-          throw new \InvalidArgumentException('Unable to set a value with a non-numeric delta in a list.');
-        }
-      });
-
       // Assign incoming values. Keys are renumbered to ensure 0-based
       // sequential deltas. If possible, reuse existing items rather than
       // creating new ones.
diff --git a/core/lib/Drupal/Core/TypedData/ListInterface.php b/core/lib/Drupal/Core/TypedData/ListInterface.php
index 5c6cf99..2e028fa 100644
--- a/core/lib/Drupal/Core/TypedData/ListInterface.php
+++ b/core/lib/Drupal/Core/TypedData/ListInterface.php
@@ -80,7 +80,7 @@ public function set($index, $value);
   public function first();
 
   /**
-   * Appends a new item to the list
+   * Appends a new item to the list.
    *
    * @param mixed $value
    *   The value of the new item.
diff --git a/core/lib/Drupal/Core/TypedData/Plugin/DataType/ItemList.php b/core/lib/Drupal/Core/TypedData/Plugin/DataType/ItemList.php
index f5b44c3..ca4131b 100644
--- a/core/lib/Drupal/Core/TypedData/Plugin/DataType/ItemList.php
+++ b/core/lib/Drupal/Core/TypedData/Plugin/DataType/ItemList.php
@@ -65,12 +65,6 @@ public function setValue($values, $notify = TRUE) {
       if (!is_array($values)) {
         throw new \InvalidArgumentException('Cannot set a list with a non-array value.');
       }
-      array_walk($values, function ($value, $index) {
-        if (!is_numeric($index)) {
-          throw new \InvalidArgumentException('Unable to set a value with a non-numeric delta in a list.');
-        }
-      });
-
       // Assign incoming values. Keys are renumbered to ensure 0-based
       // sequential deltas. If possible, reuse existing items rather than
       // creating new ones.
@@ -113,7 +107,7 @@ public function get($index) {
       throw new \InvalidArgumentException('Unable to get a value with a non-numeric delta in a list.');
     }
     // Automatically create the first item for computed fields.
-    if ($index == 0 && !isset($this->list[0]) && ($this->definition->isComputed())) {
+    if ($index == 0 && !isset($this->list[0]) && $this->definition->isComputed()) {
       $this->list[0] = $this->createItem(0);
     }
     return isset($this->list[$index]) ? $this->list[$index] : NULL;
@@ -141,30 +135,32 @@ public function set($index, $value) {
    * {@inheritdoc}
    */
   public function removeItem($index) {
-    if (!is_numeric($index)) {
-      throw new \InvalidArgumentException('Unable to drop a non-numeric delta.');
-    }
     if (isset($this->list) && array_key_exists($index, $this->list)) {
       // Remove the item, and reassign deltas.
       unset($this->list[$index]);
       $this->rekey($index);
     }
+    else {
+      throw new \InvalidArgumentException('Unable to remove item at non-existing index.');
+    }
     return $this;
   }
 
   /**
-   * @todo
+   * Renumbers the items in the list.
    *
    * @param int $from_index
+   *   Optionally, the index at which to start the renumbering, if it is known
+   *   that items before that can safely be skipped (for example, when removing
+   *   an item at a given index).
    */
   protected function rekey($from_index = 0) {
     // Re-key the list to maintain consecutive indexes.
     $this->list = array_values($this->list);
-    // Each item holds its own delta as a "name".
-    foreach ($this->list as $delta => $value) {
-      if ($delta >= $from_index) {
-        $value->setContext($delta, $this);
-      }
+    // Each item holds its own index as a "name", it needs to be updated
+    // according to the new list indexes.
+    for ($i = $from_index; $i < count($this->list); $i++) {
+      $this->list[$i]->setContext($i, $this);
     }
   }
 
diff --git a/core/modules/field/src/Entity/FieldStorageConfig.php b/core/modules/field/src/Entity/FieldStorageConfig.php
index 99dfe86..b7f9aa1 100644
--- a/core/modules/field/src/Entity/FieldStorageConfig.php
+++ b/core/modules/field/src/Entity/FieldStorageConfig.php
@@ -602,10 +602,18 @@ public function getOptionsProvider($property_name, FieldableEntityInterface $ent
     // If the field item class implements the interface, proxy it through.
     if (is_subclass_of($this->getFieldItemClass(), '\Drupal\Core\TypedData\OptionsProviderInterface')) {
       $items = $entity->get($this->getName());
-      // If the field is empty, create an item so that we can use it as the
-      // "options provider".
-      // @todo we should clear it afterwards ?
-      return $items->first() ?: $items->appendItem();
+      // We need to return a field item to be used as the "options provider". If
+      // the current entity already has such an item, use that one.
+      if ($item = $items->first()) {
+        return $item;
+      }
+      // If not, create a new item that has the entity as the parent, but do not
+      // modify the actual entity by leaving it there.
+      else {
+        $item = $items->appendItem();
+        $items->removeItem($item->getName());
+        return $item;
+      }
     }
     // @todo: Allow setting custom options provider, see
     // https://www.drupal.org/node/2002138.
diff --git a/core/modules/field_ui/src/Form/FieldStorageEditForm.php b/core/modules/field_ui/src/Form/FieldStorageEditForm.php
index 557ffc8..ff4c76b 100644
--- a/core/modules/field_ui/src/Form/FieldStorageEditForm.php
+++ b/core/modules/field_ui/src/Form/FieldStorageEditForm.php
@@ -8,7 +8,6 @@
 namespace Drupal\field_ui\Form;
 
 use Drupal\Core\Entity\EntityManagerInterface;
-use Drupal\Core\Field\FieldItemListInterface;
 use Drupal\Core\Field\FieldStorageDefinitionInterface;
 use Drupal\Core\Form\FormBase;
 use Drupal\Core\Form\FormStateInterface;
diff --git a/core/modules/hal/src/Normalizer/FieldItemNormalizer.php b/core/modules/hal/src/Normalizer/FieldItemNormalizer.php
index b437c1d..53c544c 100644
--- a/core/modules/hal/src/Normalizer/FieldItemNormalizer.php
+++ b/core/modules/hal/src/Normalizer/FieldItemNormalizer.php
@@ -106,7 +106,7 @@ protected function createTranslatedInstance(FieldItemInterface $item, $langcode)
     $delta = $item->getName();
     unset($items[$delta]);
 
-    // instead, create a new item for the entity in the requested language.
+    // Instead, create a new item for the entity in the requested language.
     $entity_translation = $item->getEntity()->getTranslation($langcode);
     $field_name = $item->getFieldDefinition()->getName();
     return $entity_translation->get($field_name)->appendItem();