diff --git a/src/Symfony/Component/Config/Resource/ReflectionClassResource.php b/src/Symfony/Component/Config/Resource/ReflectionClassResource.php
index a86a37ed8d..0ece672216 100644
--- a/src/Symfony/Component/Config/Resource/ReflectionClassResource.php
+++ b/src/Symfony/Component/Config/Resource/ReflectionClassResource.php
@@ -140,7 +140,7 @@ class ReflectionClassResource implements SelfCheckingResourceInterface, \Seriali
 
             foreach ($class->getProperties(\ReflectionProperty::IS_PUBLIC | \ReflectionProperty::IS_PROTECTED) as $p) {
                 yield $p->getDocComment().$p;
-                yield print_r($defaults[$p->name], true);
+                yield print_r($defaults[$p->name] ?? null, true);
             }
         }
 
diff --git a/src/Symfony/Component/Config/Tests/Resource/ReflectionClassResourceTest.php b/src/Symfony/Component/Config/Tests/Resource/ReflectionClassResourceTest.php
index 23f2cc567c..1f0bcb17b4 100644
--- a/src/Symfony/Component/Config/Tests/Resource/ReflectionClassResourceTest.php
+++ b/src/Symfony/Component/Config/Tests/Resource/ReflectionClassResourceTest.php
@@ -137,6 +137,14 @@ EOPHP;
         yield [1, 13, 'protected function prot($a = [123]) {}'];
         yield [0, 14, '/** priv docblock */'];
         yield [0, 15, ''];
+
+        if (\PHP_VERSION_ID >= 70400) {
+            // PHP7.4 typed properties without default value are
+            // undefined, make sure this doesn't throw an error
+            yield [1, 5, 'public array $pub;'];
+            yield [0, 7, 'protected int $prot;'];
+            yield [0, 9, 'private string $priv;'];
+        }
     }
 
     public function testEventSubscriber()