diff --git a/src/Symfony/Bundle/FrameworkBundle/CacheWarmer/ConfigBuilderCacheWarmer.php b/src/Symfony/Bundle/FrameworkBundle/CacheWarmer/ConfigBuilderCacheWarmer.php
index 7e039ae2f45bb..1ef1eaa1e6af3 100644
--- a/src/Symfony/Bundle/FrameworkBundle/CacheWarmer/ConfigBuilderCacheWarmer.php
+++ b/src/Symfony/Bundle/FrameworkBundle/CacheWarmer/ConfigBuilderCacheWarmer.php
@@ -45,7 +45,7 @@ public function warmUp(string $cacheDir /* , string $buildDir = null */): array
         $buildDir = 1 < \func_num_args() ? func_get_arg(1) : null;
 
         if (!$buildDir) {
-            return [];
+            $buildDir = $this->kernel->getBuildDir();
         }
 
         $generator = new ConfigBuilderGenerator($buildDir);
diff --git a/src/Symfony/Bundle/FrameworkBundle/Tests/CacheWarmer/ConfigBuilderCacheWarmerTest.php b/src/Symfony/Bundle/FrameworkBundle/Tests/CacheWarmer/ConfigBuilderCacheWarmerTest.php
index a0e8fe834ef81..5ba9dae41a161 100644
--- a/src/Symfony/Bundle/FrameworkBundle/Tests/CacheWarmer/ConfigBuilderCacheWarmerTest.php
+++ b/src/Symfony/Bundle/FrameworkBundle/Tests/CacheWarmer/ConfigBuilderCacheWarmerTest.php
@@ -90,7 +90,7 @@ public function process(ContainerBuilder $container): void
         $warmer = new ConfigBuilderCacheWarmer($kernel);
         $warmer->warmUp($kernel->getCacheDir());
 
-        self::assertDirectoryDoesNotExist($kernel->getBuildDir().'/Symfony');
+        self::assertDirectoryExists($kernel->getBuildDir().'/Symfony');
         self::assertDirectoryDoesNotExist($kernel->getCacheDir().'/Symfony');
 
         $warmer->warmUp($kernel->getCacheDir(), $kernel->getBuildDir());