diff --git a/rdpy/network/type.py b/rdpy/network/type.py
index b641744..0dacec9 100644
--- a/rdpy/network/type.py
+++ b/rdpy/network/type.py
@@ -74,7 +74,7 @@ class Type(object):
         self.__read__(s)
         #check constant value
         if self._constant and old != self:
-            raise InvalidExpectedDataException("const value expected")
+            raise InvalidExpectedDataException("%s const value expected %s != %s"%(self.__class__, old.value, self.value))
         
     def __read__(self, s):
         '''
diff --git a/rdpy/protocol/rdp/mcs.py b/rdpy/protocol/rdp/mcs.py
index 2d0fd8d..3f0d78d 100644
--- a/rdpy/protocol/rdp/mcs.py
+++ b/rdpy/protocol/rdp/mcs.py
@@ -51,7 +51,7 @@ class MCS(LayerAutomata):
         ctor call base class ctor
         @param presentation: presentation layer
         '''
-        LayerAutomata.__init__(self, presentation)
+        LayerAutomata.__init__(self, presentation._mode, presentation)
         self._clientSettings = gcc.ClientSettings()
         self._serverSettings = gcc.ServerSettings()
         #default user Id
diff --git a/rdpy/protocol/rdp/tpkt.py b/rdpy/protocol/rdp/tpkt.py
index 2100872..32af92e 100644
--- a/rdpy/protocol/rdp/tpkt.py
+++ b/rdpy/protocol/rdp/tpkt.py
@@ -13,7 +13,7 @@ class TPKT(RawLayer):
     #first byte of classic tpkt header
     TPKT_PACKET = UInt8(3)
     
-    def __init__(self, presentation = None):
+    def __init__(self, presentation):
         '''
         Constructor
         '''