diff --git a/Tools/autotest/pysim/util.py b/Tools/autotest/pysim/util.py
index 910208339e..97f382f247 100644
--- a/Tools/autotest/pysim/util.py
+++ b/Tools/autotest/pysim/util.py
@@ -375,9 +375,11 @@ def start_MAVProxy_SITL(atype, aircraft=None, setup=False, master='tcp:127.0.0.1
     local_mp_modules_dir = os.path.abspath(
         os.path.join(__file__, '..', '..', '..', 'mavproxy_modules'))
     env = dict(os.environ)
-    env['PYTHONPATH'] = (local_mp_modules_dir +
-                         os.pathsep +
-                         env.get('PYTHONPATH', ''))
+    old = env.get('PYTHONPATH', None)
+    env['PYTHONPATH'] = local_mp_modules_dir
+    if old is not None:
+        env['PYTHONPATH'] += os.path.pathsep + old
+
     import pexpect
     global close_list
     cmd = []
@@ -392,6 +394,7 @@ def start_MAVProxy_SITL(atype, aircraft=None, setup=False, master='tcp:127.0.0.1
     cmd.extend(options)
     cmd.extend(['--default-modules', 'misc,terrain,wp,rally,fence,param,arm,mode,rc,cmdlong,output'])
 
+    print("PYTHONPATH: %s" % str(env['PYTHONPATH']))
     print("Running: %s" % cmd_as_shell(cmd))
 
     ret = pexpect.spawn(cmd[0], cmd[1:], logfile=logfile, encoding=ENCODING, timeout=60, env=env)