diff --git a/tests/test-hgweb-json.t b/tests/test-hgweb-json.t
--- a/tests/test-hgweb-json.t
+++ b/tests/test-hgweb-json.t
@@ -2201,7 +2201,7 @@
 Commit message with null character
 
   $ echo foo >> da/foo
-  >>> open('msg', 'wb').write('commit with null character: \0\n') and None
+  >>> open('msg', 'wb').write(b'commit with null character: \0\n') and None
   $ hg ci -l msg
   $ rm msg