diff --git a/ChangeLog b/ChangeLog
index 2dc47051c7..15c9cdfded 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -34,6 +34,7 @@ phpMyAdmin - ChangeLog
 - issue #16982 Do not try to delete internal relations if they are not configured
 - issue #16982 Show success messages on Designer for add and remove relation operations
 - issue        Fixed possible "Undefined index: clause_is_unique" on replace value in cell
+- issue #16991 Fixed case where $_SERVER['REQUEST_METHOD'] is undefined
 
 5.1.1 (2021-06-04)
 - issue #13325 Fixed created procedure shows up in triggers and events and vice-versa
diff --git a/libraries/classes/Controllers/LogoutController.php b/libraries/classes/Controllers/LogoutController.php
index 7794b1d9d9..a814cf62d9 100644
--- a/libraries/classes/Controllers/LogoutController.php
+++ b/libraries/classes/Controllers/LogoutController.php
@@ -12,7 +12,7 @@ class LogoutController
     {
         global $auth_plugin, $token_mismatch;
 
-        if ($_SERVER['REQUEST_METHOD'] !== 'POST' || $token_mismatch) {
+        if (($_SERVER['REQUEST_METHOD'] ?? 'GET') !== 'POST' || $token_mismatch) {
             Core::sendHeaderLocation('./index.php?route=/');
 
             return;
diff --git a/libraries/classes/Core.php b/libraries/classes/Core.php
index bc98a43dd4..83ee4e5e6c 100644
--- a/libraries/classes/Core.php
+++ b/libraries/classes/Core.php
@@ -1393,7 +1393,7 @@ class Core
         $token_mismatch = true;
         $token_provided = false;
 
-        if ($_SERVER['REQUEST_METHOD'] !== 'POST') {
+        if (($_SERVER['REQUEST_METHOD'] ?? 'GET') !== 'POST') {
             return;
         }
 
diff --git a/setup/index.php b/setup/index.php
index 76e0941db2..8439b1159c 100644
--- a/setup/index.php
+++ b/setup/index.php
@@ -56,7 +56,7 @@ if ($page === 'config') {
 
 if ($page === 'servers') {
     $controller = new ServersController($GLOBALS['ConfigFile'], new Template());
-    if (isset($_GET['mode']) && $_GET['mode'] === 'remove' && $_SERVER['REQUEST_METHOD'] === 'POST') {
+    if (isset($_GET['mode']) && $_GET['mode'] === 'remove' && ($_SERVER['REQUEST_METHOD'] ?? 'GET') === 'POST') {
         $controller->destroy([
             'id' => $_GET['id'] ?? null,
         ]);