diff --git a/gunicorn/workers/workertmp.py b/gunicorn/workers/workertmp.py
index ba53af2a..3d6af9eb 100644
--- a/gunicorn/workers/workertmp.py
+++ b/gunicorn/workers/workertmp.py
@@ -46,7 +46,7 @@ class WorkerTmp(object):
         except AttributeError:
             # python < 2.6
             self._tmp.truncate(0)
-            os.write(self._tmp.fileno(), "X")
+            os.write(self._tmp.fileno(), b"X")
 
     def last_update(self):
         return os.fstat(self._tmp.fileno()).st_ctime