diff --git a/composer.json b/composer.json
index 63108b1e132e9..7f98fbd213517 100644
--- a/composer.json
+++ b/composer.json
@@ -49,7 +49,8 @@
         "symfony/polyfill-mbstring": "~1.0",
         "symfony/polyfill-php72": "~1.5",
         "symfony/polyfill-php73": "^1.11",
-        "symfony/polyfill-php80": "^1.15"
+        "symfony/polyfill-php80": "^1.15",
+        "symfony/polyfill-php81": "^1.22"
     },
     "replace": {
         "symfony/asset": "self.version",
diff --git a/src/Symfony/Component/Config/Definition/PrototypedArrayNode.php b/src/Symfony/Component/Config/Definition/PrototypedArrayNode.php
index 68bb270172eb6..72d3578be7a09 100644
--- a/src/Symfony/Component/Config/Definition/PrototypedArrayNode.php
+++ b/src/Symfony/Component/Config/Definition/PrototypedArrayNode.php
@@ -225,11 +225,11 @@ protected function normalizeValue($value)
 
         $value = $this->remapXml($value);
 
-        $isAssoc = array_keys($value) !== range(0, \count($value) - 1);
+        $isList = array_is_list($value);
         $normalized = [];
         foreach ($value as $k => $v) {
             if (null !== $this->keyAttribute && \is_array($v)) {
-                if (!isset($v[$this->keyAttribute]) && \is_int($k) && !$isAssoc) {
+                if (!isset($v[$this->keyAttribute]) && \is_int($k) && $isList) {
                     $ex = new InvalidConfigurationException(sprintf('The attribute "%s" must be set for path "%s".', $this->keyAttribute, $this->getPath()));
                     $ex->setPath($this->getPath());
 
@@ -271,7 +271,7 @@ protected function normalizeValue($value)
             }
 
             $prototype = $this->getPrototypeForChild($k);
-            if (null !== $this->keyAttribute || $isAssoc) {
+            if (null !== $this->keyAttribute || !$isList) {
                 $normalized[$k] = $prototype->normalize($v);
             } else {
                 $normalized[] = $prototype->normalize($v);
@@ -304,9 +304,10 @@ protected function mergeValues($leftSide, $rightSide)
             return $rightSide;
         }
 
+        $isList = array_is_list($rightSide);
         foreach ($rightSide as $k => $v) {
-            // prototype, and key is irrelevant, append the element
-            if (null === $this->keyAttribute) {
+            // prototype, and key is irrelevant there are no named keys, append the element
+            if (null === $this->keyAttribute && $isList) {
                 $leftSide[] = $v;
                 continue;
             }
diff --git a/src/Symfony/Component/Config/Tests/Definition/PrototypedArrayNodeTest.php b/src/Symfony/Component/Config/Tests/Definition/PrototypedArrayNodeTest.php
index 7a58ead8da967..58d2408a71c90 100644
--- a/src/Symfony/Component/Config/Tests/Definition/PrototypedArrayNodeTest.php
+++ b/src/Symfony/Component/Config/Tests/Definition/PrototypedArrayNodeTest.php
@@ -338,4 +338,56 @@ public function getDataForKeyRemovedLeftValueOnly()
            ],
         ];
     }
+
+    /**
+     * @dataProvider getPrototypedArrayNodeDataToMerge
+     */
+    public function testPrototypedArrayNodeMerge($left, $right, $expected)
+    {
+        $node = new PrototypedArrayNode('options');
+        $node->setNormalizeKeys(false);
+        $node->setPrototype(new VariableNode('value'));
+        $node->setDefaultValue([]);
+
+        $result = $node->merge($left, $right);
+
+        self::assertSame($result, $expected);
+    }
+
+    public function getPrototypedArrayNodeDataToMerge()
+    {
+        return [
+            // data to merged is a plain array
+            [
+                ['foo', 'bar'],
+                ['foo', 'baz', 'qux'],
+                ['foo', 'bar', 'foo', 'baz', 'qux'],
+            ],
+            // data to be merged is an associative array
+            [
+                ['option1' => true, 'option2' => 'foo'],
+                [
+                    'option2' => 'bar',
+                    'option3' => 42,
+                    'option4' => [
+                        'option41' => 'baz',
+                        'option42' => [
+                            'option423' => 'qux',
+                        ],
+                    ],
+                ],
+                [
+                    'option1' => true,
+                    'option2' => 'bar',
+                    'option3' => 42,
+                    'option4' => [
+                        'option41' => 'baz',
+                        'option42' => [
+                            'option423' => 'qux',
+                        ],
+                    ],
+                ],
+            ],
+        ];
+    }
 }
diff --git a/src/Symfony/Component/Config/composer.json b/src/Symfony/Component/Config/composer.json
index d71373bb8151f..13e4cd409f3ac 100644
--- a/src/Symfony/Component/Config/composer.json
+++ b/src/Symfony/Component/Config/composer.json
@@ -18,7 +18,8 @@
     "require": {
         "php": ">=7.1.3",
         "symfony/filesystem": "^3.4|^4.0|^5.0",
-        "symfony/polyfill-ctype": "~1.8"
+        "symfony/polyfill-ctype": "~1.8",
+        "symfony/polyfill-php81": "^1.22"
     },
     "require-dev": {
         "symfony/event-dispatcher": "^3.4|^4.0|^5.0",

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