diff --git a/tests/heartbeats/characterize.py b/tests/heartbeats/characterize.py
index ab35b3d106ca..abb7f1ba65b6 100644
--- a/tests/heartbeats/characterize.py
+++ b/tests/heartbeats/characterize.py
@@ -65,7 +65,7 @@ def get_command(build_target, layout_thread_count, renderer, page, profile):
     """Get the command to execute.
     """
     return path.join(TOP_DIR, "target", build_target, "servo") + \
-        " -p %d -x -y %d %s -Z profile-script-events '%s'" % \
+        " -p %d -o output.png -y %d %s -Z profile-script-events '%s'" % \
         (profile, layout_thread_count, renderer, page)