diff --git a/lib/bup/main.py b/lib/bup/main.py
index 226e463fd029..903d28cbfb9d 100755
--- a/lib/bup/main.py
+++ b/lib/bup/main.py
@@ -198,6 +198,8 @@ fix_stdout = not already_fixed and os.isatty(1)
 fix_stderr = not already_fixed and os.isatty(2)
 
 if fix_stdout or fix_stderr:
+    _ttymask = (1 if helpers.istty1 else 0) | (2 if helpers.istty2 else 0)
+    os.environ['BUP_FORCE_TTY'] = str(_ttymask)
     tty_env = merge_dict(environ,
                          {b'BUP_FORCE_TTY': (b'%d'
                                              % ((fix_stdout and 1 or 0)