diff --git a/infra/build/functions/trial_build.py b/infra/build/functions/trial_build.py
index e3a1b7af2..7f593bfa3 100644
--- a/infra/build/functions/trial_build.py
+++ b/infra/build/functions/trial_build.py
@@ -287,7 +287,7 @@ def wait_on_builds(build_ids, credentials, cloud_project, end_time):  # pylint:
     # Update status every hour.
     if current_time >= next_check_time:
       logging.info(f'[{current_time}] Remaining builds: '
-                   '{len(wait_builds)}, {wait_builds}')
+                   f'{len(wait_builds)}, {wait_builds}')
       next_check_time += datetime.timedelta(hours=1)
 
     # Warn users and write a summary if build is about to end.