diff --git a/tests/run-tests.py b/tests/run-tests.py
--- a/tests/run-tests.py
+++ b/tests/run-tests.py
@@ -1352,7 +1352,9 @@
         if PYTHON3:
             session = session.encode('ascii')
         def toggletrace(cmd):
-            quoted = shellquote(cmd.strip()).replace(b'\\', b'\\\\')
+            cmd = cmd.decode('utf8')
+            quoted = shellquote(cmd.strip()).encode('utf8')
+            quoted = quoted.replace(b'\\', b'\\\\')
             if active:
                 script.append(
                     b'echo END %s %s >> "$HGCATAPULTSERVERPIPE"\n' % (