diff --git a/Mac/Unsupported/mactcp/socket.py b/Mac/Unsupported/mactcp/socket.py
index 0a1b5f0..e6600b1 100644
--- a/Mac/Unsupported/mactcp/socket.py
+++ b/Mac/Unsupported/mactcp/socket.py
@@ -216,6 +216,7 @@
 class _socketfile:
 	
 	def __init__(self, sock, rw, bs):
+		if rw[1:] == 'b': rw = rw[:1]
 		if rw not in ('r', 'w'): raise _myerror, "mode must be 'r' or 'w'"
 		self.sock = sock
 		self.rw = rw