diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,3 +1,7 @@
+## 0.10.0.1
+
+* Instance for Data.Sequence.Seq is incorrect. [#83](https://github.com/snoyberg/mono-traversable/issues/83)
+
 ## 0.10.0
 
 * Remove `Functor` instance for `MinLen` [#82](https://github.com/snoyberg/mono-traversable/issues/82)
diff --git a/mono-traversable.cabal b/mono-traversable.cabal
--- a/mono-traversable.cabal
+++ b/mono-traversable.cabal
@@ -1,5 +1,5 @@
 name:                mono-traversable
-version:             0.10.0
+version:             0.10.0.1
 synopsis:            Type classes for mapping, folding, and traversing monomorphic containers
 description:         Monomorphic variants of the Functor, Foldable, and Traversable typeclasses. If you understand Haskell's basic typeclasses, you understand mono-traversable. In addition to what you are used to, it adds on an IsSequence typeclass and has code for marking data structures as non-empty.
 homepage:            https://github.com/snoyberg/mono-traversable
diff --git a/src/Data/MonoTraversable.hs b/src/Data/MonoTraversable.hs
--- a/src/Data/MonoTraversable.hs
+++ b/src/Data/MonoTraversable.hs
@@ -571,7 +571,7 @@
     {-# INLINE omapM_ #-}
 instance MonoFoldable (Tree a)
 instance MonoFoldable (Seq a) where
-    headEx = flip Seq.index 1
+    headEx = flip Seq.index 0
     lastEx xs = Seq.index xs (Seq.length xs - 1)
     {-# INLINE headEx #-}
     {-# INLINE lastEx #-}
@@ -1297,4 +1297,3 @@
                        EmptyR  -> Seq.empty
                        ys :> y -> ys Seq.|> y
         ) :> f w
-
diff --git a/test/Spec.hs b/test/Spec.hs
--- a/test/Spec.hs
+++ b/test/Spec.hs
@@ -457,3 +457,6 @@
         it "#31 find doesn't infinitely loop on NonEmpty" $
             find (== "a") ("a" NE.:| ["d","fgf"]) @?= Just "a"
 
+        it "#83 head on Seq works correctly" $ do
+            headEx (Seq.fromList [1 :: Int,2,3]) @?= (1 :: Int)
+            headMay (Seq.fromList [] :: Seq.Seq Int) @?= Nothing