diff --git a/lib/locking.py b/lib/locking.py
index 38f65df..f527a9f 100644
--- a/lib/locking.py
+++ b/lib/locking.py
@@ -357,7 +357,7 @@
     """
     return os.read(self.read_fd, size)
 
-  def Post(self, data='!'):
+  def Post(self, data=b'!'):
     """Write |data| to the pipe.
 
     Args: