Minor
diff --git a/Lib/fontTools/ttLib/__init__.py b/Lib/fontTools/ttLib/__init__.py
index 9b852a4..b4f4ff0 100644
--- a/Lib/fontTools/ttLib/__init__.py
+++ b/Lib/fontTools/ttLib/__init__.py
@@ -761,7 +761,7 @@
 	table, but it's nice to present it as such in the TTX format.
 	"""
 	
-	def __init__(self, tag):
+	def __init__(self, tag=None):
 		pass
 	
 	def toXML(self, writer, ttFont):