diff --git a/core/modules/options/options.module b/core/modules/options/options.module
index 3c649c5..d21c320 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 +445,25 @@ function options_options_list(FieldDefinitionInterface $field_definition, Entity
   return options_allowed_values($field_definition, $entity);
 }
 
+/**
+ * Flattens an array of allowed values.
+ *
+ * @param $array
+ *   A single or multidimensional array.
+ * @return
+ *   A flattened array.
+ */
+function options_array_flatten($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;
+}