diff --git a/barcode/ean.py b/barcode/ean.py
index c89408f..dafe929 100755
--- a/barcode/ean.py
+++ b/barcode/ean.py
@@ -90,9 +90,9 @@ def calculate_checksum(self):
         def sum_(x, y):
             return int(x) + int(y)
 
-        evensum = reduce(sum_, self.ean[-2::-2])
-        oddsum = reduce(sum_, self.ean[-1::-2])
-        return (10 - ((evensum + oddsum * 3) % 10)) % 10
+        evensum = reduce(sum_, self.ean[1:12:2])
+        oddsum = reduce(sum_, self.ean[0:11:2])
+        return (10 - ((oddsum + evensum * 3) % 10)) % 10
 
     def build(self):
         """Builds the barcode pattern from `self.ean`.