diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,15 @@
 Changelog
 =========
 
+Version 0.3.4.2
+----------
+
+*August 25, 2021*
+
+<https://github.com/mstksg/nonempty-containers/releases/tag/v0.3.4.2>
+
+*   Compatibility with GHC 9. (@andremarianiello)
+
 Version 0.3.4.1
 ----------
 
diff --git a/nonempty-containers.cabal b/nonempty-containers.cabal
--- a/nonempty-containers.cabal
+++ b/nonempty-containers.cabal
@@ -4,10 +4,10 @@
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: da5f8ed9003a7dab25c32dfaa59717a40a3575aeebb7ddaa8f6f97613d435220
+-- hash: af05370c135085af4252c99e5575aa4ba9ebe2bf244c46bffaee55772a7ccb53
 
 name:           nonempty-containers
-version:        0.3.4.1
+version:        0.3.4.2
 synopsis:       Non-empty variants of containers data types, with full API
 description:    Efficient and optimized non-empty versions of types from /containers/.
                 Inspired by /non-empty-containers/ library, except attempting a more
diff --git a/src/Data/IntMap/NonEmpty/Internal.hs b/src/Data/IntMap/NonEmpty/Internal.hs
--- a/src/Data/IntMap/NonEmpty/Internal.hs
+++ b/src/Data/IntMap/NonEmpty/Internal.hs
@@ -181,7 +181,7 @@
     1 -> k (z fromList)
     _ -> error "gunfold"
   dataTypeOf _   = intMapDataType
-  dataCast1      = gcast1
+  dataCast1 f    = gcast1 f
 
 fromListConstr :: Constr
 fromListConstr = mkConstr intMapDataType "fromList" [] Prefix
@@ -562,10 +562,17 @@
 -- elements in order of ascending keys, while 'IntMap' traverses positive
 -- keys first, then negative keys.
 instance Foldable1 NEIntMap where
+#if MIN_VERSION_base(4,11,0)
+    fold1 (NEIntMap _ v m) = maybe v (v <>)
+                           . F.foldMap Just
+                           . M.elems
+                           $ m
+#else
     fold1 (NEIntMap _ v m) = option v (v <>)
                            . F.foldMap (Option . Just)
                            . M.elems
                            $ m
+#endif
     {-# INLINE fold1 #-}
     foldMap1 f = foldMapWithKey (const f)
     {-# INLINE foldMap1 #-}
diff --git a/src/Data/Map/NonEmpty/Internal.hs b/src/Data/Map/NonEmpty/Internal.hs
--- a/src/Data/Map/NonEmpty/Internal.hs
+++ b/src/Data/Map/NonEmpty/Internal.hs
@@ -182,7 +182,7 @@
       1 -> k (z fromList)
       _ -> error "gunfold"
     dataTypeOf _   = mapDataType
-    dataCast2      = gcast2
+    dataCast2 f    = gcast2 f
 
 fromListConstr :: Constr
 fromListConstr = mkConstr mapDataType "fromList" [] Prefix
@@ -275,9 +275,15 @@
     => (k -> a -> m)
     -> NEMap k a
     -> m
+#if MIN_VERSION_base(4,11,0)
+foldMapWithKey f (NEMap k0 v m) = maybe (f k0 v) (f k0 v <>)
+                                . M.foldMapWithKey (\k -> Just . f k)
+                                $ m
+#else
 foldMapWithKey f (NEMap k0 v m) = option (f k0 v) (f k0 v <>)
                                 . M.foldMapWithKey (\k -> Option . Just . f k)
                                 $ m
+#endif
 {-# INLINE foldMapWithKey #-}
 
 -- | /O(n)/. Map a function over all values in the map.
@@ -545,9 +551,15 @@
 
 -- | Traverses elements in order of ascending keys
 instance Foldable1 (NEMap k) where
+#if MIN_VERSION_base(4,11,0)
+    fold1 (NEMap _ v m) = maybe v (v <>)
+                        . F.foldMap Just
+                        $ m
+#else
     fold1 (NEMap _ v m) = option v (v <>)
                         . F.foldMap (Option . Just)
                         $ m
+#endif
     {-# INLINE fold1 #-}
     foldMap1 f = foldMapWithKey (const f)
     {-# INLINE foldMap1 #-}
diff --git a/src/Data/Sequence/NonEmpty/Internal.hs b/src/Data/Sequence/NonEmpty/Internal.hs
--- a/src/Data/Sequence/NonEmpty/Internal.hs
+++ b/src/Data/Sequence/NonEmpty/Internal.hs
@@ -185,7 +185,7 @@
     gunfold k z _   = k (k (z (:<||)))
     toConstr _      = consConstr
     dataTypeOf _    = seqDataType
-    dataCast1       = gcast1
+    dataCast1 f     = gcast1 f
 
 consConstr :: Constr
 consConstr  = mkConstr seqDataType ":<||" [] Infix
@@ -317,9 +317,15 @@
 -- a folding function that also depends on the element's index, and applies
 -- it to every element in the sequence.
 foldMapWithIndex :: Semigroup m => (Int -> a -> m) -> NESeq a -> m
+#if MIN_VERSION_base(4,11,0)
+foldMapWithIndex f (x :<|| xs) = maybe (f 0 x) (f 0 x <>)
+                               . Seq.foldMapWithIndex (\i -> Just . f (i + 1))
+                               $ xs
+#else
 foldMapWithIndex f (x :<|| xs) = option (f 0 x) (f 0 x <>)
                                . Seq.foldMapWithIndex (\i -> Option . Just . f (i + 1))
                                $ xs
+#endif
 {-# INLINE foldMapWithIndex #-}
 
 -- | /O(n)/. 'traverseWithIndex1' is a version of 'traverse1' that also
@@ -467,9 +473,15 @@
     {-# INLINE length #-}
 
 instance Foldable1 NESeq where
+#if MIN_VERSION_base(4,11,0)
+    fold1 (x :<|| xs) = maybe x (x <>)
+                      . F.foldMap Just
+                      $ xs
+#else
     fold1 (x :<|| xs) = option x (x <>)
                       . F.foldMap (Option . Just)
                       $ xs
+#endif
     {-# INLINE fold1 #-}
     foldMap1 f = foldMapWithIndex (const f)
     {-# INLINE foldMap1 #-}
diff --git a/src/Data/Set/NonEmpty/Internal.hs b/src/Data/Set/NonEmpty/Internal.hs
--- a/src/Data/Set/NonEmpty/Internal.hs
+++ b/src/Data/Set/NonEmpty/Internal.hs
@@ -153,7 +153,7 @@
     1 -> k (z fromList)
     _ -> error "gunfold"
   dataTypeOf _   = setDataType
-  dataCast1      = gcast1
+  dataCast1 f    = gcast1 f
 
 fromListConstr :: Constr
 fromListConstr = mkConstr setDataType "fromList" [] Prefix
@@ -377,6 +377,17 @@
 
 -- | Traverses elements in ascending order
 instance Foldable1 NESet where
+#if MIN_VERSION_base(4,11,0)
+    fold1 (NESet x s) = maybe x (x <>)
+                      . F.foldMap Just
+                      $ s
+    {-# INLINE fold1 #-}
+    -- TODO: benchmark against maxView-based method
+    foldMap1 f (NESet x s) = maybe (f x) (f x <>)
+                           . F.foldMap (Just . f)
+                           $ s
+    {-# INLINE foldMap1 #-}
+#else
     fold1 (NESet x s) = option x (x <>)
                       . F.foldMap (Option . Just)
                       $ s
@@ -386,6 +397,7 @@
                            . F.foldMap (Option . Just . f)
                            $ s
     {-# INLINE foldMap1 #-}
+#endif
     toNonEmpty = toList
     {-# INLINE toNonEmpty #-}