diff --git a/src/Data/FunctorWithIndex.lua b/src/Data/FunctorWithIndex.lua
index f5f1730..7109441 100644
--- a/src/Data/FunctorWithIndex.lua
+++ b/src/Data/FunctorWithIndex.lua
@@ -3,7 +3,7 @@ return {
     return function(xs)
       local l = #xs
       local result = {}
-      for i = 1, l do result[i] = f(i)(xs[i]) end
+      for i = 1, l do result[i] = f(i - 1)(xs[i]) end
       return result
     end
   end)
diff --git a/src/Data/Traversable.lua b/src/Data/Traversable.lua
index a846178..68a708e 100644
--- a/src/Data/Traversable.lua
+++ b/src/Data/Traversable.lua
@@ -25,7 +25,7 @@ return {
               end
             end
 
-            return go(0, #array)
+            return go(1, #array + 1)
           end
         end
       end