diff --git a/core/modules/language/src/Form/NegotiationUrlForm.php b/core/modules/language/src/Form/NegotiationUrlForm.php
index fa1cd3e..2ed158f 100644
--- a/core/modules/language/src/Form/NegotiationUrlForm.php
+++ b/core/modules/language/src/Form/NegotiationUrlForm.php
@@ -9,6 +9,7 @@
 
 use Drupal\Core\Form\ConfigFormBase;
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Language\LanguageInterface;
 use Drupal\Core\Language\LanguageManagerInterface;
 use Drupal\Core\Config\ConfigFactoryInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
@@ -85,7 +86,7 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       '#tree' => TRUE,
       '#title' => $this->t('Path prefix configuration'),
       '#open' => TRUE,
-      '#description' => $this->t('Language codes or other custom text to use as a path prefix for URL language detection. For the default language, this value may be left blank. <strong>Modifying this value may break existing URLs. Use with caution in a production environment.</strong> Example: Specifying "deutsch" as the path prefix code for German results in URLs like "example.com/deutsch/contact".'),
+      '#description' => $this->t('Language codes or other custom text to use as a path prefix for URL language detection. For the selected fallback language, this value may be left blank. <strong>Modifying this value may break existing URLs. Use with caution in a production environment.</strong> Example: Specifying "deutsch" as the path prefix code for German results in URLs like "example.com/deutsch/contact".'),
       '#states' => array(
         'visible' => array(
           ':input[name="language_negotiation_url_part"]' => array(
@@ -142,14 +143,17 @@ public function validateForm(array &$form, FormStateInterface $form_state) {
 
     // Count repeated values for uniqueness check.
     $count = array_count_values($form_state->getValue('prefix'));
+    $default_langcode = $this->config('language.negotiation')->get('selected_langcode');
+    if ($default_langcode == LanguageInterface::LANGCODE_SITE_DEFAULT) {
+      $default_langcode = $this->languageManager->getDefaultLanguage()->getId();
+    }
     foreach ($languages as $langcode => $language) {
       $value = $form_state->getValue(array('prefix', $langcode));
-
       if ($value === '') {
-        if (!$language->isDefault() && $form_state->getValue('language_negotiation_url_part') == LanguageNegotiationUrl::CONFIG_PATH_PREFIX) {
+        if (!($default_langcode == $langcode) && $form_state->getValue('language_negotiation_url_part') == LanguageNegotiationUrl::CONFIG_PATH_PREFIX) {
           // Throw a form error if the prefix is blank for a non-default language,
           // although it is required for selected negotiation type.
-          $form_state->setErrorByName("prefix][$langcode", $this->t('The prefix may only be left blank for the default language.'));
+          $form_state->setErrorByName("prefix][$langcode", $this->t('The prefix may only be left blank for the selected negotiation fallback language.'));
         }
       }
       elseif (strpos($value, '/') !== FALSE) {
diff --git a/core/modules/language/src/Tests/LanguageConfigurationTest.php b/core/modules/language/src/Tests/LanguageConfigurationTest.php
index a5efb2f..7e865db 100644
--- a/core/modules/language/src/Tests/LanguageConfigurationTest.php
+++ b/core/modules/language/src/Tests/LanguageConfigurationTest.php
@@ -83,13 +83,16 @@ function testLanguageConfiguration() {
     $this->drupalPostForm(NULL, $edit, t('Save configuration'));
     $this->assertFieldByXPath('//input[@name="prefix[fr]"]', 'french', 'French path prefix has changed.');
 
-    // Check that prefix of non default language cannot be changed to
-    // empty string.
+
+    // Change default negotiation language.
+    $this->config('language.negotiation')->set('selected_langcode', 'fr')->save();
+    // Check that the prefix of a language that is not the negotiation one
+    // cannot be changed to empty string.
     $edit = array(
       'prefix[en]' => '',
     );
     $this->drupalPostForm(NULL, $edit, t('Save configuration'));
-    $this->assertText(t('The prefix may only be left blank for the default language.'), 'English prefix cannot be changed to empty string.');
+    $this->assertText(t('The prefix may only be left blank for the selected negotiation fallback language.'));
 
     //  Check that prefix cannot be changed to contain a slash.
     $edit = array(
diff --git a/core/modules/language/src/Tests/LanguageSwitchingTest.php b/core/modules/language/src/Tests/LanguageSwitchingTest.php
index 2dc0777..1c39a88 100644
--- a/core/modules/language/src/Tests/LanguageSwitchingTest.php
+++ b/core/modules/language/src/Tests/LanguageSwitchingTest.php
@@ -161,7 +161,7 @@ protected function doTestLanguageBlockAnonymous($block_label) {
   }
 
   /**
-   * Test languge switcher links for domain based negotiation
+   * Test language switcher links for domain based negotiation.
    */
   function testLanguageBlockWithDomain() {
     // Add the Italian language.