diff --git a/gunicorn/arbiter.py b/gunicorn/arbiter.py
index af0209e1..340ea299 100644
--- a/gunicorn/arbiter.py
+++ b/gunicorn/arbiter.py
@@ -273,7 +273,7 @@ class Arbiter(object):
         Wake up the arbiter by writing to the PIPE
         """
         try:
-            os.write(self.PIPE[1], '.')
+            os.write(self.PIPE[1], b'.')
         except IOError as e:
             if e.errno not in [errno.EAGAIN, errno.EINTR]:
                 raise