diff --git a/src/vs/base/browser/dom.ts b/src/vs/base/browser/dom.ts
index bbb81a0b407afa959c9dfb1e6a94cce052ed6c32..7bbfbb3040b96d1423de95962ba96785e881811e 100644
--- a/src/vs/base/browser/dom.ts
+++ b/src/vs/base/browser/dom.ts
@@ -1434,7 +1434,7 @@ export namespace WebFileSystemAccess {
 	}
 
 	export function supported(obj: any & Window): obj is FileSystemAccess {
-		const candidate = obj as FileSystemAccess;
+		const candidate = obj as FileSystemAccess | undefined;
 		if (typeof candidate?.showDirectoryPicker === 'function') {
 			return true;
 		}