diff --git a/core/modules/language/language.admin.inc b/core/modules/language/language.admin.inc
index e17988d..ca1d288 100644
--- a/core/modules/language/language.admin.inc
+++ b/core/modules/language/language.admin.inc
@@ -746,6 +746,11 @@ function language_negotiation_configure_url_form_validate($form, &$form_state) {
         form_error($form['prefix'][$langcode], t('The prefix may only be left blank for the default language.'));
       }
     }
+    elseif (strpos($value, '/') !== FALSE) {
+      // Throw a form error if the strings contains a slash,
+      // which would not work.
+      form_error($form['prefix'][$langcode], t('The prefix may not contain a slash.'));
+    }
     elseif (isset($count[$value]) && $count[$value] > 1) {
       // Throw a form error if there are two languages with the same
       // domain/prefix.
diff --git a/core/modules/language/lib/Drupal/language/Tests/LanguageConfigurationTest.php b/core/modules/language/lib/Drupal/language/Tests/LanguageConfigurationTest.php
index 0372e1b..76af730 100644
--- a/core/modules/language/lib/Drupal/language/Tests/LanguageConfigurationTest.php
+++ b/core/modules/language/lib/Drupal/language/Tests/LanguageConfigurationTest.php
@@ -41,40 +41,62 @@ class LanguageConfigurationTest extends WebTestBase {
 
     // Check if the Default English language has no path prefix.
     $this->drupalGet('admin/config/regional/language/detection/url');
-    $this->assertFieldByXPath('//input[@name="prefix[en]"]', '', t('Default English has no path prefix.'));
+    $this->assertFieldByXPath('//input[@name="prefix[en]"]', '', 'Default English has no path prefix.');
 
     // Add predefined language.
     $edit = array(
       'predefined_langcode' => 'fr',
     );
-    $this->drupalPost('admin/config/regional/language/add', $edit, t('Add language'));
+    $this->drupalPost('admin/config/regional/language/add', $edit, 'Add language');
     $this->assertText('French');
-    $this->assertEqual($this->getUrl(), url('admin/config/regional/language', array('absolute' => TRUE)), t('Correct page redirection.'));
+    $this->assertEqual($this->getUrl(), url('admin/config/regional/language', array('absolute' => TRUE)), 'Correct page redirection.');
 
     // Check if the Default English language has no path prefix.
     $this->drupalGet('admin/config/regional/language/detection/url');
-    $this->assertFieldByXPath('//input[@name="prefix[en]"]', '', t('Default English has no path prefix.'));
+    $this->assertFieldByXPath('//input[@name="prefix[en]"]', '', 'Default English has no path prefix.');
     // Check if French has a path prefix.
     $this->drupalGet('admin/config/regional/language/detection/url');
-    $this->assertFieldByXPath('//input[@name="prefix[fr]"]', 'fr', t('French has a path prefix.'));
+    $this->assertFieldByXPath('//input[@name="prefix[fr]"]', 'fr', 'French has a path prefix.');
 
     // Check if we can change the default language.
     $this->drupalGet('admin/config/regional/language');
-    $this->assertFieldChecked('edit-site-default-en', t('English is the default language.'));
+    $this->assertFieldChecked('edit-site-default-en', 'English is the default language.');
     // Change the default language.
     $edit = array(
       'site_default' => 'fr',
     );
     $this->drupalPost(NULL, $edit, t('Save configuration'));
-    $this->assertNoFieldChecked('edit-site-default-en', t('Default language updated.'));
-    $this->assertEqual($this->getUrl(), url('admin/config/regional/language', array('absolute' => TRUE)), t('Correct page redirection.'));
+    $this->assertNoFieldChecked('edit-site-default-en', 'Default language updated.');
+    $this->assertEqual($this->getUrl(), url('admin/config/regional/language', array('absolute' => TRUE)), 'Correct page redirection.');
 
     // Check if a valid language prefix is added afrer changing the default
     // language.
     $this->drupalGet('admin/config/regional/language/detection/url');
-    $this->assertFieldByXPath('//input[@name="prefix[en]"]', 'en', t('A valid path prefix has been added to the previous default language.'));
+    $this->assertFieldByXPath('//input[@name="prefix[en]"]', 'en', 'A valid path prefix has been added to the previous default language.');
     // Check if French still has a path prefix.
     $this->drupalGet('admin/config/regional/language/detection/url');
-    $this->assertFieldByXPath('//input[@name="prefix[fr]"]', 'fr', t('French still has a path prefix.'));
+    $this->assertFieldByXPath('//input[@name="prefix[fr]"]', 'fr', 'French still has a path prefix.');
+
+    // Check that prefix can be changed.
+    $edit = array(
+      'prefix[fr]' => 'french',
+    );
+    $this->drupalPost(NULL, $edit, t('Save configuration'));
+    $this->assertFieldByXPath('//input[@name="prefix[fr]"]', 'french', 'French path prefix has changed.');
+
+    // Check that prefix of non default langauge cannot be changed to
+    // empty string.
+    $edit = array(
+      'prefix[en]' => '',
+    );
+    $this->drupalPost(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.');
+
+    //  Check that prefix cannot be changed to contain a slash.
+    $edit = array(
+      'prefix[en]' => 'foo/bar',
+    );
+    $this->drupalPost(NULL, $edit, t('Save configuration'));
+    $this->assertText(t('The prefix may not contain a slash.'), 'English prefix cannot be changed to contain a slash.');
   }
 }