diff --git a/ip.py b/ip.py
index e4832a1..02acd1e 100644
--- a/ip.py
+++ b/ip.py
@@ -213,9 +213,9 @@ class TCP_Recreate:
                             0,      # Header checksum
                             sip,
                             dip)
-        shorts = struct.unpack('!hhhhhhhhhh', iphdr)
+        shorts = struct.unpack('!HHHHHHHHHH', iphdr)
         shsum = sum(shorts)
-        ipsum = struct.pack('!h', (~shsum & 0xffff) - 2) # -2? WTF?
+        ipsum = struct.pack('!H', ((shsum + (shsum >> 16)) & 0xffff) ^ 0xffff)
         iphdr = iphdr[:10] + ipsum + iphdr[12:]
 
         tcphdr = struct.pack('!HHLLBBHHH',