diff --git a/saslauthd/__init__.py b/saslauthd/__init__.py
--- a/saslauthd/__init__.py
+++ b/saslauthd/__init__.py
@@ -268,13 +274,13 @@
             if success:
                 # #1170: Catch broken pipe error (incomplete authentication request)
                 try:
-                    clientsocket.send(struct.pack("!H2s", 2, "OK"))
+                    clientsocket.send(struct.pack("!H2s", 2, b"OK"))
                 except:
                     pass
             else:
                 # #1170: Catch broken pipe error (incomplete authentication request)
                 try:
-                    clientsocket.send(struct.pack("!H2s", 2, "NO"))
+                    clientsocket.send(struct.pack("!H2s", 2, b"NO"))
                 except:
                     pass