diff --git a/src/google/adk/runners.py b/src/google/adk/runners.py
index 017997bb3..9456a5f4b 100644
--- a/src/google/adk/runners.py
+++ b/src/google/adk/runners.py
@@ -20,6 +20,7 @@
 from typing import AsyncGenerator
 from typing import Generator
 from typing import Optional
+from typing import Union
 import warnings
 
 from google.genai import types