diff --git a/changelog.md b/changelog.md
--- a/changelog.md
+++ b/changelog.md
@@ -2,6 +2,10 @@
 
 ## WIP
 
+## [1.5.0] - 2021-08-25
+
+- Change indexes used by `accum` from `Int` to `Finite`.
+
 ## [1.4.4] - 2021-06-26
 
 - Add `ix'`
diff --git a/package.yaml b/package.yaml
--- a/package.yaml
+++ b/package.yaml
@@ -1,5 +1,5 @@
 name: vector-sized
-version: 1.4.4
+version: 1.5.0
 synopsis: Size tagged vectors
 description: Please see README.md
 category: Data
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
@@ -927,9 +927,10 @@
 accum :: VG.Vector v a
       => (a -> b -> a) -- ^ accumulating function @f@
       -> Vector v m a  -- ^ initial vector (of length @m@)
-      -> [(Int,b)]     -- ^ list of index/value pairs (of length @n@)
+      -> [(Finite m,b)]     -- ^ list of index/value pairs (of length @n@)
       -> Vector v m a
-accum f (Vector v) us = Vector (VG.accum f v us)
+accum f (Vector v) us =
+  Vector (VG.accum f v $ (fmap . first) (fromIntegral . getFinite) us)
 {-# inline accum #-}
 
 -- | /O(m+n)/ For each pair @(i,b)@ from the vector of pairs, replace the vector
diff --git a/src/Data/Vector/Primitive/Sized.hs b/src/Data/Vector/Primitive/Sized.hs
--- a/src/Data/Vector/Primitive/Sized.hs
+++ b/src/Data/Vector/Primitive/Sized.hs
@@ -706,7 +706,7 @@
 accum :: Prim a
       => (a -> b -> a) -- ^ accumulating function @f@
       -> Vector m a  -- ^ initial vector (of length @m@)
-      -> [(Int,b)]     -- ^ list of index/value pairs (of length @n@)
+      -> [(Finite m,b)]     -- ^ list of index/value pairs (of length @n@)
       -> Vector m a
 accum = V.accum
 {-# inline accum #-}
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
@@ -741,7 +741,7 @@
 -- > accum (+) <5,9,2> [(2,4),(1,6),(0,3),(1,7)] = <5+3, 9+6+7, 2+4>
 accum :: (a -> b -> a) -- ^ accumulating function @f@
       -> Vector m a  -- ^ initial vector (of length @m@)
-      -> [(Int,b)]     -- ^ list of index/value pairs (of length @n@)
+      -> [(Finite m,b)]     -- ^ list of index/value pairs (of length @n@)
       -> Vector m a
 accum = V.accum
 {-# inline accum #-}
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
@@ -761,7 +761,7 @@
 accum :: Storable a
       => (a -> b -> a) -- ^ accumulating function @f@
       -> Vector m a  -- ^ initial vector (of length @m@)
-      -> [(Int,b)]     -- ^ list of index/value pairs (of length @n@)
+      -> [(Finite m,b)]     -- ^ list of index/value pairs (of length @n@)
       -> Vector m a
 accum = V.accum
 {-# inline accum #-}
diff --git a/src/Data/Vector/Unboxed/Sized.hs b/src/Data/Vector/Unboxed/Sized.hs
--- a/src/Data/Vector/Unboxed/Sized.hs
+++ b/src/Data/Vector/Unboxed/Sized.hs
@@ -762,7 +762,7 @@
 accum :: Unbox a
       => (a -> b -> a) -- ^ accumulating function @f@
       -> Vector m a  -- ^ initial vector (of length @m@)
-      -> [(Int,b)]     -- ^ list of index/value pairs (of length @n@)
+      -> [(Finite m,b)]     -- ^ list of index/value pairs (of length @n@)
       -> Vector m a
 accum = V.accum
 {-# inline accum #-}
diff --git a/vector-sized.cabal b/vector-sized.cabal
--- a/vector-sized.cabal
+++ b/vector-sized.cabal
@@ -1,13 +1,13 @@
 cabal-version: 1.12
 
--- This file has been generated from package.yaml by hpack version 0.33.0.
+-- This file has been generated from package.yaml by hpack version 0.34.4.
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: e36af7d31e151c5e8d7ad5636dd5d31b47e82d6fe6384f8f76753ed0e17b61b1
+-- hash: 8cc33c7ff1b35236dcaed143ba951337eb4f12e3c53c616fa15ec2a073c0942b
 
 name:           vector-sized
-version:        1.4.4
+version:        1.5.0
 synopsis:       Size tagged vectors
 description:    Please see README.md
 category:       Data