diff --git a/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php b/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php
index dffca66710..520a886deb 100644
--- a/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php
+++ b/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php
@@ -309,7 +309,7 @@ EOF;
             $this->asFiles = false;
 
             if ($this->preload && null !== $autoloadFile = $this->getAutoloadFile()) {
-                $autoloadFile = substr($this->export($autoloadFile), 2, -1);
+                $autoloadFile = trim($this->export($autoloadFile), '()\\');
 
                 $code[$options['class'].'.preload.php'] = <<<EOF
 <?php
@@ -2076,9 +2076,7 @@ EOF;
 
     private function getAutoloadFile(): ?string
     {
-        if (null === $this->targetDirRegex) {
-            return null;
-        }
+        $file = null;
 
         foreach (spl_autoload_functions() as $autoloader) {
             if (!\is_array($autoloader)) {
@@ -2097,14 +2095,14 @@ EOF;
                 if (0 === strpos($class, 'ComposerAutoloaderInit') && $class::getLoader() === $autoloader[0]) {
                     $file = \dirname((new \ReflectionClass($class))->getFileName(), 2).'/autoload.php';
 
-                    if (preg_match($this->targetDirRegex.'A', $file)) {
+                    if (null !== $this->targetDirRegex && preg_match($this->targetDirRegex.'A', $file)) {
                         return $file;
                     }
                 }
             }
         }
 
-        return null;
+        return $file;
     }
 
     private function getClasses(Definition $definition): array