diff --git a/rank-product.cabal b/rank-product.cabal
--- a/rank-product.cabal
+++ b/rank-product.cabal
@@ -1,5 +1,5 @@
 name:                rank-product
-version:             0.1.0.2
+version:             0.1.0.3
 synopsis:            Find the rank product of a data set.
 description:         Find the rank product of a data set and get the p-value from a permutation test.
 homepage:            http://github.com/GregorySchwartz/rank-product#readme
diff --git a/src/Statistics/RankProduct.hs b/src/Statistics/RankProduct.hs
--- a/src/Statistics/RankProduct.hs
+++ b/src/Statistics/RankProduct.hs
@@ -35,7 +35,11 @@
 -- | Get the rank product of a list of Entity.
 rankProduct :: [Entity] -> [RankProductEntity]
 rankProduct xs =
-    fmap (RankProductEntity . (** (1 / genericLength xs)) . fromIntegral . product)
+    fmap ( RankProductEntity
+         . (** (1 / (genericLength . head . fmap unEntity $ xs)))
+         . fromIntegral
+         . product
+         )
         . transpose
         . fmap rankList
         . transpose