diff --git a/Tools/autotest/sim_vehicle.py b/Tools/autotest/sim_vehicle.py
index 13953db8be..480ce5ed7e 100755
--- a/Tools/autotest/sim_vehicle.py
+++ b/Tools/autotest/sim_vehicle.py
@@ -728,7 +728,7 @@ def start_mavproxy(opts, stuff):
     old = env.get('PYTHONPATH', None)
     env['PYTHONPATH'] = local_mp_modules_dir
     if old is not None:
-        env['PYTHONPATH'] += os.path.sep + old
+        env['PYTHONPATH'] += os.path.pathsep + old
 
     run_cmd_blocking("Run MavProxy", cmd, env=env)
     progress("MAVProxy exited")