diff --git a/telegram.rb b/telegram.rb
index 7ba56cd..6c77270 100644
--- a/telegram.rb
+++ b/telegram.rb
@@ -102,9 +102,10 @@ class Telegram
     to_return.data = bytes[3..-3]
     # parse checksum (CS)
     checksum = (bytes[-2]<<8)+bytes[-1]
+    bytes[0..-3].each { |b| checksum -= b }
     # run some checks
     raise "wrong length. expected #{length}, got #{to_return.data.length-1}" if (to_return.transmission==SEND or to_return.transmission==ANSWER) and length != to_return.data.length-1
-    raise "wrong checksum. expected #{to_return.checksum}, got #{checksum}" if checksum != to_return.checksum
+    raise "wrong checksum. off by #{checksum}" if checksum != 0
     return to_return
   end