diff --git a/src/google/adk/runners.py b/src/google/adk/runners.py
index 4b418a3a..8fd34eee 100644
--- a/src/google/adk/runners.py
+++ b/src/google/adk/runners.py
@@ -268,9 +268,15 @@ class Runner:
 
     Yields:
       The events generated by the agent.
+
+    Raises:
+      ValueError: If the session is not found.
     """
     run_config = run_config or RunConfig()
 
+    if not new_message.role:
+      new_message.role = 'user'
+
     async def _run_with_trace(
         new_message: types.Content,
     ) -> AsyncGenerator[Event, None]: