diff --git a/src/Symfony/Component/AssetMapper/Factory/MappedAssetFactory.php b/src/Symfony/Component/AssetMapper/Factory/MappedAssetFactory.php
index 1d2ba703e6592..14f273b7b474d 100644
--- a/src/Symfony/Component/AssetMapper/Factory/MappedAssetFactory.php
+++ b/src/Symfony/Component/AssetMapper/Factory/MappedAssetFactory.php
@@ -128,6 +128,6 @@ private function isVendor(string $sourcePath): bool
         $sourcePath = realpath($sourcePath);
         $vendorDir = realpath($this->vendorDir);
 
-        return $sourcePath && str_starts_with($sourcePath, $vendorDir);
+        return $sourcePath && $vendorDir && str_starts_with($sourcePath, $vendorDir);
     }
 }
diff --git a/src/Symfony/Component/AssetMapper/Tests/Factory/MappedAssetFactoryTest.php b/src/Symfony/Component/AssetMapper/Tests/Factory/MappedAssetFactoryTest.php
index d4e129a50ccfc..7e3d1641e36a8 100644
--- a/src/Symfony/Component/AssetMapper/Tests/Factory/MappedAssetFactoryTest.php
+++ b/src/Symfony/Component/AssetMapper/Tests/Factory/MappedAssetFactoryTest.php
@@ -26,6 +26,8 @@
 
 class MappedAssetFactoryTest extends TestCase
 {
+    private const DEFAULT_FIXTURES = __DIR__.'/../Fixtures/assets/vendor';
+
     private AssetMapperInterface&MockObject $assetMapper;
 
     public function testCreateMappedAsset()
@@ -137,7 +139,15 @@ public function testCreateMappedAssetInVendor()
         $this->assertTrue($asset->isVendor);
     }
 
-    private function createFactory(?AssetCompilerInterface $extraCompiler = null): MappedAssetFactory
+    public function testCreateMappedAssetInMissingVendor()
+    {
+        $assetMapper = $this->createFactory(null, '/this-path-does-not-exist/');
+        $asset = $assetMapper->createMappedAsset('lodash.js', __DIR__.'/../Fixtures/assets/vendor/lodash/lodash.index.js');
+        $this->assertSame('lodash.js', $asset->logicalPath);
+        $this->assertFalse($asset->isVendor);
+    }
+
+    private function createFactory(?AssetCompilerInterface $extraCompiler = null, ?string $vendorDir = self::DEFAULT_FIXTURES): MappedAssetFactory
     {
         $compilers = [
             new JavaScriptImportPathCompiler($this->createMock(ImportMapConfigReader::class)),
@@ -162,7 +172,7 @@ private function createFactory(?AssetCompilerInterface $extraCompiler = null): M
         $factory = new MappedAssetFactory(
             $pathResolver,
             $compiler,
-            __DIR__.'/../Fixtures/assets/vendor',
+            $vendorDir,
         );
 
         // mock the AssetMapper to behave like normal: by calling back to the factory