diff --git a/core/modules/options/lib/Drupal/options/Tests/OptionsArrayFlattenUnitTest.php b/core/modules/options/lib/Drupal/options/Tests/OptionsArrayFlattenUnitTest.php
new file mode 100644
index 0000000..259c5d5
--- /dev/null
+++ b/core/modules/options/lib/Drupal/options/Tests/OptionsArrayFlattenUnitTest.php
@@ -0,0 +1,43 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\options\Tests\OptionsFieldUITest.
+ */
+
+namespace Drupal\options\Tests;
+
+use Drupal\simpletest\DrupalUnitTestBase;
+
+/**
+ * Tests the array flatten utility function.
+ */
+class OptionsArrayFlattenUnitTest extends DrupalUnitTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getInfo() {
+    return array(
+      'name' => 'Options flatten array',
+        'description' => 'Test multi-dimensional array is flattened.',
+        'group' => 'Field types',
+      );
+  }
+
+  public function testArrayFlatten() {
+    $p = array('v1' => 'l1', 'g1' => array('v2' => 'l2', 'v3' => 'l3'));
+    $sub = options_array_flatten($p);
+    $this->assertEqual(array('v1' => 'l1', 'v2' => 'l2', 'v3' => 'l3'), $sub);
+
+    $p = array (
+        'parent1' => array(3 => 'child1', 4 => 'child2', 5 => 'child3'),
+        'parent2' => array(8 => 'child1', 9 => 'child2')
+    );
+    $sub = options_array_flatten($p);
+    $this->assertEqual(array(
+        3 => 'child1', 4 => 'child2', 5 => 'child3',
+        8 => 'child1', 9 => 'child2'), $sub
+    );
+  }
+}
diff --git a/core/modules/options/options.module b/core/modules/options/options.module
index 3c649c5..e69c541 100644
--- a/core/modules/options/options.module
+++ b/core/modules/options/options.module
@@ -414,7 +414,8 @@ function options_field_validate(EntityInterface $entity = NULL, $field, $instanc
     $entity = _field_create_entity_from_ids($ids);
   }
 
-  $allowed_values = options_allowed_values($instance, $entity);
+  // Flatten the array before validating to account for optgroups.
+  $allowed_values = options_array_flatten(options_allowed_values($field, $instance, $entity));
   foreach ($items as $delta => $item) {
     if (!empty($item['value'])) {
       if (!empty($allowed_values) && !isset($allowed_values[$item['value']])) {
@@ -444,3 +445,26 @@ function options_options_list(FieldDefinitionInterface $field_definition, Entity
   return options_allowed_values($field_definition, $entity);
 }
 
+/**
+ * Flattens an array of allowed values.
+ *
+ * @param array $array
+ *   A single or multidimensional array
+ *
+ * @return array
+ *   A flattened array
+ */
+function options_array_flatten($array) {
+  $result = array();
+  if (is_array($array)) {
+    foreach ($array as $key => $value) {
+      if (is_array($value)) {
+        $result += options_array_flatten($value);
+      }
+      else {
+        $result[$key] = $value;
+      }
+    }
+  }
+  return $result;
+}