diff --git a/node/mock-test.ts b/node/mock-test.ts
index 56e432879..01a5b23b3 100644
--- a/node/mock-test.ts
+++ b/node/mock-test.ts
@@ -262,7 +262,7 @@ export class MockTestRunner {
         }
 
         // Write marker to indicate download completed.
-        const marker = downloadDestination + '.completed';
+        const marker = path.join(downloadDestination, '.completed');
         fs.writeFileSync(marker, '');
         return downloadPath
     }