diff --git a/core/config/schema/core.data_types.schema.yml b/core/config/schema/core.data_types.schema.yml
index ab6b944..2690ffa 100644
--- a/core/config/schema/core.data_types.schema.yml
+++ b/core/config/schema/core.data_types.schema.yml
@@ -769,6 +769,7 @@ entity_reference_selection:
     target_bundles:
       type: sequence
       label: 'types'
+      nullable: true
       sequence:
         type: string
         label: 'Type'
diff --git a/core/lib/Drupal/Core/Config/Schema/ArrayElement.php b/core/lib/Drupal/Core/Config/Schema/ArrayElement.php
index 88914c0..fb89063 100644
--- a/core/lib/Drupal/Core/Config/Schema/ArrayElement.php
+++ b/core/lib/Drupal/Core/Config/Schema/ArrayElement.php
@@ -186,4 +186,14 @@ public function setTypedConfig(TypedConfigManagerInterface $typed_config) {
     $this->typedConfig = $typed_config;
   }
 
+  /**
+   * Determines if this element allows NULL as a value.
+   *
+   * @return bool
+   *   TRUE if NULL is a valid value, FALSE otherwise.
+   */
+  public function isNullable() {
+    return isset($this->definition['nullable']) && $this->definition['nullable'] == TRUE;
+  }
+
 }
diff --git a/core/lib/Drupal/Core/Config/Schema/SchemaCheckTrait.php b/core/lib/Drupal/Core/Config/Schema/SchemaCheckTrait.php
index e1ce8dc..2193a55 100644
--- a/core/lib/Drupal/Core/Config/Schema/SchemaCheckTrait.php
+++ b/core/lib/Drupal/Core/Config/Schema/SchemaCheckTrait.php
@@ -106,9 +106,13 @@ protected function checkValue($key, $value) {
           (($type == 'double' || $type == 'integer') && $element instanceof FloatInterface) ||
           ($type == 'boolean' && $element instanceof BooleanInterface) ||
           ($type == 'string' && $element instanceof StringInterface) ||
-          // Null values are allowed for all types.
+          // Null values are allowed for all primitive types.
           ($value === NULL);
       }
+      // Array elements can also opt-in for allowing a NULL value.
+      elseif ($element instanceof ArrayElement && $element->isNullable() && $value === NULL) {
+        $success = TRUE;
+      }
       $class = get_class($element);
       if (!$success) {
         return array($error_key => "variable type is $type but applied schema class is $class");
diff --git a/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/SelectionBase.php b/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/SelectionBase.php
index c851f14..3f8bccd 100644
--- a/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/SelectionBase.php
+++ b/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/SelectionBase.php
@@ -111,6 +111,9 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
 
     // Merge-in default values.
     $selection_handler_settings += array(
+      // For the 'target_bundles' setting, a NULL value is equivalent to "allow
+      // entities from any bundle to be referenced" and an empty array value is
+      // equivalent to "no entities from any bundle can be referenced".
       'target_bundles' => NULL,
       'sort' => array(
         'field' => '_none',
@@ -128,7 +131,7 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
         '#type' => 'checkboxes',
         '#title' => $this->t('Bundles'),
         '#options' => $bundle_options,
-        '#default_value' => $selection_handler_settings['target_bundles'],
+        '#default_value' => (array) $selection_handler_settings['target_bundles'],
         '#required' => TRUE,
         '#size' => 6,
         '#multiple' => TRUE,
@@ -336,7 +339,7 @@ protected function buildEntityQuery($match = NULL, $match_operator = 'CONTAINS')
 
     // If the 'target_bundles' setting is an empty array, force the query to
     // never return anything and bail out early.
-    if ($handler_settings['target_bundles'] === []) {
+    if (isset($handler_settings['target_bundles']) && $handler_settings['target_bundles'] === []) {
       $query->condition($entity_type->getKey('id'), NULL, '=');
 
       return $query;
diff --git a/core/modules/system/src/Tests/Entity/EntityReferenceSelection/EntityReferenceSelectionAccessTest.php b/core/modules/system/src/Tests/Entity/EntityReferenceSelection/EntityReferenceSelectionAccessTest.php
index 274df65..46cc952 100644
--- a/core/modules/system/src/Tests/Entity/EntityReferenceSelection/EntityReferenceSelectionAccessTest.php
+++ b/core/modules/system/src/Tests/Entity/EntityReferenceSelection/EntityReferenceSelectionAccessTest.php
@@ -77,7 +77,7 @@ public function testNodeHandler() {
       'target_type' => 'node',
       'handler' => 'default',
       'handler_settings' => array(
-        'target_bundles' => array(),
+        'target_bundles' => NULL,
       ),
     );
 
@@ -203,7 +203,7 @@ public function testUserHandler() {
       'target_type' => 'user',
       'handler' => 'default',
       'handler_settings' => array(
-        'target_bundles' => array(),
+        'target_bundles' => NULL,
         'include_anonymous' => TRUE,
       ),
     );
@@ -345,7 +345,7 @@ public function testCommentHandler() {
       'target_type' => 'comment',
       'handler' => 'default',
       'handler_settings' => array(
-        'target_bundles' => array(),
+        'target_bundles' => NULL,
       ),
     );
 
diff --git a/core/modules/system/src/Tests/Entity/EntityReferenceSelection/EntityReferenceSelectionSortTest.php b/core/modules/system/src/Tests/Entity/EntityReferenceSelection/EntityReferenceSelectionSortTest.php
index 98be261..3d0d4d1 100644
--- a/core/modules/system/src/Tests/Entity/EntityReferenceSelection/EntityReferenceSelectionSortTest.php
+++ b/core/modules/system/src/Tests/Entity/EntityReferenceSelection/EntityReferenceSelectionSortTest.php
@@ -100,7 +100,7 @@ public function testSort() {
       'target_type' => 'node',
       'handler' => 'default',
       'handler_settings' => array(
-        'target_bundles' => array(),
+        'target_bundles' => NULL,
         // Add sorting.
         'sort' => array(
           'field' => 'field_text.value',