diff --git a/testing/mozharness/mozharness/base/vcs/mercurial.py b/testing/mozharness/mozharness/base/vcs/mercurial.py
index b3ca8f5de6db..8362cc5d7fe2 100755
--- a/testing/mozharness/mozharness/base/vcs/mercurial.py
+++ b/testing/mozharness/mozharness/base/vcs/mercurial.py
@@ -388,8 +388,9 @@ class MercurialVCS(ScriptMixin, LogMixin, TransferMixin):
 
         parser = RepositoryUpdateRevisionParser(config=self.config,
                                                 log_obj=self.log_obj)
-        if self.run_command(args, output_parser=parser,
-                            partial_env={'PYTHONUNBUFFERED': '1'}):
+        env = dict(os.environ)
+        env[b'PYTHONUNBUFFERED'] = b'1'
+        if self.run_command(args, output_parser=parser, env=env):
             raise VCSException('repo checkout failed!')
 
         if not parser.revision: