diff --git a/luigi/mock.py b/luigi/mock.py
index 5e781f7751..bf158eb011 100644
--- a/luigi/mock.py
+++ b/luigi/mock.py
@@ -124,15 +124,10 @@ def set_wrapper(self, wrapper):
                 self.wrapper = wrapper
 
             def write(self, data):
-                if six.PY3:
-                    stderrbytes = sys.stderr.buffer
-                else:
-                    stderrbytes = sys.stderr
-
                 if mock_target._mirror_on_stderr:
                     if self._write_line:
                         sys.stderr.write(fn + ": ")
-                    stderrbytes.write(data)
+                    sys.stderr.write(data)
                     if (data[-1]) == '\n':
                         self._write_line = True
                     else: