diff --git a/changelog.md b/changelog.md
--- a/changelog.md
+++ b/changelog.md
@@ -1,5 +1,8 @@
 # Change Log
 
+## [0.4.0.0] - 2016-11-01
+- Correct type signature on `index'`
+
 ## [0.3.3.0] - 2016-08-10
 - Add instances for Data, Typeable and Generic
 
diff --git a/src/Data/Vector/Generic/Sized.hs b/src/Data/Vector/Generic/Sized.hs
--- a/src/Data/Vector/Generic/Sized.hs
+++ b/src/Data/Vector/Generic/Sized.hs
@@ -297,7 +297,7 @@
 
 -- | /O(1)/ Safe indexing using a 'Proxy'.
 index' :: forall v n m a. (KnownNat n, KnownNat m, VG.Vector v a)
-       => Vector v (n+m) a -> Proxy n -> a
+       => Vector v (n+m+1) a -> Proxy n -> a
 index' (Vector v) p = v `VG.unsafeIndex` i
   where i = fromInteger (natVal p)
 {-# inline index' #-}
diff --git a/src/Data/Vector/Sized.hs b/src/Data/Vector/Sized.hs
--- a/src/Data/Vector/Sized.hs
+++ b/src/Data/Vector/Sized.hs
@@ -258,7 +258,7 @@
 
 -- | /O(1)/ Safe indexing using a 'Proxy'.
 index' :: forall n m a. (KnownNat n, KnownNat m)
-       => Vector (n+m) a -> Proxy n -> a
+       => Vector (n+m+1) a -> Proxy n -> a
 index' = V.index'
 {-# inline index' #-}
 
diff --git a/src/Data/Vector/Storable/Sized.hs b/src/Data/Vector/Storable/Sized.hs
--- a/src/Data/Vector/Storable/Sized.hs
+++ b/src/Data/Vector/Storable/Sized.hs
@@ -259,7 +259,7 @@
 
 -- | /O(1)/ Safe indexing using a 'Proxy'.
 index' :: forall n m a. (KnownNat n, KnownNat m, Storable a)
-       => Vector (n+m) a -> Proxy n -> a
+       => Vector (n+m+1) a -> Proxy n -> a
 index' = V.index'
 {-# inline index' #-}
 
diff --git a/vector-sized.cabal b/vector-sized.cabal
--- a/vector-sized.cabal
+++ b/vector-sized.cabal
@@ -1,5 +1,5 @@
 name:                vector-sized
-version:             0.3.3.0
+version:             0.4.0.0
 synopsis:            Size tagged vectors
 description:         Please see README.md
 homepage:            http://github.com/expipiplus1/vector-sized#readme