diff --git a/Data/Header.hs b/Data/Header.hs
--- a/Data/Header.hs
+++ b/Data/Header.hs
@@ -6,10 +6,9 @@
 
 import Data.Binary (encode, Binary)
 import qualified Data.ByteString.Lazy as B
-import Data.CSum as C
 
 -- |A class of network headers that assumes a checksum is present.
-class (Binary h) => L3Header h a c | h -> a, a -> h, h -> c where
+class (Num c,Binary h) => L3Header h a c | h -> a, a -> h, h -> c where
 	-- |Returns the checksum from the header
 	getChecksum :: h -> c
 
@@ -28,7 +27,6 @@
 
 	-- |Computes the checksum
 	computeChecksum :: h -> c
-	computeChecksum h = C.csum16 (encode (zeroChecksum h))
 
 	-- |Computes the checksum, returns a header with the proper checksum
 	fillChecksum :: h -> h
@@ -40,14 +38,14 @@
 
 	-- |Returns True iff the checksum is valid
 	valid :: h -> Bool
-	valid h = zeroCSum == getChecksum h || computeChecksum h == getChecksum h
+	valid h = computeChecksum h == getChecksum h
 
 -- |A class of network addresses that assumes there is a 'broadcast' concept.
 class (Binary a) => L3Address a h | a -> h, h -> a where
 	localBroadcast :: a -> a
 	globalBroadcast :: a
 
-class (Binary a) => L4Header h p where
-	fixChecksum :: L3Header => h -> l3 -> h
+class (Binary h, Binary p) => L4Header h p | h -> p where
+	fixChecksum :: (L3Header l3 a c) => h -> l3 -> h
 	srcPort	:: h -> p
 	dstPort :: h -> p
diff --git a/Data/IP.hs b/Data/IP.hs
--- a/Data/IP.hs
+++ b/Data/IP.hs
@@ -120,8 +120,9 @@
 		put (src h)
 		put (dst h)
 		putWord8 0
-		pw8 (protocol h)
-		pw16 (payloadLength h))
+		pW8 $ fromIntegral (protocol h)
+		pW16 (payloadLength h))
+	computeChecksum h = csum16 (encode (zeroChecksum h))
 
 instance L3Address IPv4 IPv4Header where
 	localBroadcast (IPv4 a) = IPv4 $ B.concat [B.pack [0xFF], B.drop 1 a]
diff --git a/network-data.cabal b/network-data.cabal
--- a/network-data.cabal
+++ b/network-data.cabal
@@ -1,5 +1,5 @@
 name:		network-data
-version:	0.0.1
+version:	0.0.2
 license:	BSD3
 license-file:	LICENSE
 author:		Thomas DuBuisson <thomas.dubuisson@gmail.com>