diff --git a/core/lib/Drupal/Core/Installer/Form/SelectLanguageForm.php b/core/lib/Drupal/Core/Installer/Form/SelectLanguageForm.php
index 55fb7b64ac821545a0c34feef6cbae3a42904699..bd61dab1dc7e1a91de600ce8c2ffe387922a28ae 100644
--- a/core/lib/Drupal/Core/Installer/Form/SelectLanguageForm.php
+++ b/core/lib/Drupal/Core/Installer/Form/SelectLanguageForm.php
@@ -58,7 +58,7 @@ public function buildForm(array $form, FormStateInterface $form_state, $install_
     }
     asort($select_options);
     $request = Request::createFromGlobals();
-    $browser_langcode = UserAgent::getBestMatchingLangcode($request->server->get('HTTP_ACCEPT_LANGUAGE'), $browser_options);
+    $browser_langcode = UserAgent::getBestMatchingLangcode($request->server->get('HTTP_ACCEPT_LANGUAGE', ''), $browser_options);
     $form['langcode'] = [
       '#type' => 'select',
       '#title' => 'Choose language',
diff --git a/core/modules/aggregator/tests/modules/aggregator_test/src/Controller/AggregatorTestRssController.php b/core/modules/aggregator/tests/modules/aggregator_test/src/Controller/AggregatorTestRssController.php
index af53a43ea068f2ba6210e8bb7fd5647a129b7b05..d7903ec5f091d9aae0eb32184e9f97b8678bd989 100644
--- a/core/modules/aggregator/tests/modules/aggregator_test/src/Controller/AggregatorTestRssController.php
+++ b/core/modules/aggregator/tests/modules/aggregator_test/src/Controller/AggregatorTestRssController.php
@@ -32,8 +32,8 @@ public function testFeed($use_last_modified, $use_etag, Request $request) {
     $last_modified = strtotime('Sun, 19 Nov 1978 05:00:00 GMT');
     $etag = Crypt::hashBase64($last_modified);
 
-    $if_modified_since = strtotime($request->server->get('HTTP_IF_MODIFIED_SINCE'));
-    $if_none_match = stripslashes($request->server->get('HTTP_IF_NONE_MATCH'));
+    $if_modified_since = strtotime($request->server->get('HTTP_IF_MODIFIED_SINCE', ''));
+    $if_none_match = stripslashes($request->server->get('HTTP_IF_NONE_MATCH', ''));
 
     // Send appropriate response. We respond with a 304 not modified on either
     // etag or on last modified.