diff --git a/core/modules/language/config/install/language.negotiation.yml b/core/modules/language/config/install/language.negotiation.yml
index e4e8e22..dde0bea 100644
--- a/core/modules/language/config/install/language.negotiation.yml
+++ b/core/modules/language/config/install/language.negotiation.yml
@@ -6,3 +6,4 @@ url:
     en: ''
   domains:
     en: ''
+selected_langcode: site_default
diff --git a/core/modules/language/src/Form/NegotiationUrlForm.php b/core/modules/language/src/Form/NegotiationUrlForm.php
index 1e3b55d..b304046 100644
--- a/core/modules/language/src/Form/NegotiationUrlForm.php
+++ b/core/modules/language/src/Form/NegotiationUrlForm.php
@@ -153,7 +153,9 @@ public function validateForm(array &$form, FormStateInterface $form_state) {
         if (!($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 selected detection fallback language.'));
+          $form_state->setErrorByName("prefix][$langcode", $this->t('The prefix may only be left blank for the <a href="@url">selected detection fallback language.</a>', [
+            '@url' => $this->getUrlGenerator()->generate('language.negotiation_selected'),
+          ]));
         }
       }
       elseif (strpos($value, '/') !== FALSE) {
diff --git a/core/modules/language/src/Tests/LanguageConfigurationTest.php b/core/modules/language/src/Tests/LanguageConfigurationTest.php
index 025da77..8648328 100644
--- a/core/modules/language/src/Tests/LanguageConfigurationTest.php
+++ b/core/modules/language/src/Tests/LanguageConfigurationTest.php
@@ -91,6 +91,12 @@ function testLanguageConfiguration() {
     $this->drupalPostForm(NULL, $edit, t('Save configuration'));
     $this->assertFieldByXPath('//input[@name="prefix[fr]"]', 'french', 'French path prefix has changed.');
 
+    // Check that the prefix can be removed.
+    $edit = array(
+      'prefix[fr]' => '',
+    );
+    $this->drupalPostForm(NULL, $edit, t('Save configuration'));
+    $this->assertNoText(t('The prefix may only be left blank for the selected detection fallback language.'), 'The path prefix can be removed for the default language');
 
     // Change default negotiation language.
     $this->config('language.negotiation')->set('selected_langcode', 'fr')->save();