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:             0.3.0.0
+version:             0.3.0.1
 synopsis:            Type classes for mapping, folding, and traversing monomorphic containers
 description:         Monomorphic variants of the Functor, Foldable, and Traversable typeclasses. Contains even more experimental code for abstracting containers and sequences.
 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
@@ -200,45 +200,45 @@
     ofoldr :: (Element mono -> b -> b) -> b -> mono -> b
     default ofoldr :: (t a ~ mono, a ~ Element (t a), F.Foldable t) => (Element mono -> b -> b) -> b -> mono -> b
     ofoldr = F.foldr
-    
+
     ofoldl' :: (a -> Element mono -> a) -> a -> mono -> a
     default ofoldl' :: (t b ~ mono, b ~ Element (t b), F.Foldable t) => (a -> Element mono -> a) -> a -> mono -> a
     ofoldl' = F.foldl'
 
     otoList :: mono -> [Element mono]
     otoList t = build (\ mono n -> ofoldr mono n t)
-    
+
     oall :: (Element mono -> Bool) -> mono -> Bool
     oall f = getAll . ofoldMap (All . f)
-    
+
     oany :: (Element mono -> Bool) -> mono -> Bool
     oany f = getAny . ofoldMap (Any . f)
-    
+
     onull :: mono -> Bool
     onull = oall (const False)
-    
+
     olength :: mono -> Int
     olength = ofoldl' (\i _ -> i + 1) 0
-    
+
     olength64 :: mono -> Int64
     olength64 = ofoldl' (\i _ -> i + 1) 0
-    
+
     ocompareLength :: Integral i => mono -> i -> Ordering
     ocompareLength c0 i0 = olength c0 `compare` fromIntegral i0 -- FIXME more efficient implementation
 
     otraverse_ :: (MonoFoldable mono, Applicative f) => (Element mono -> f b) -> mono -> f ()
     otraverse_ f = ofoldr ((*>) . f) (pure ())
-    
+
     ofor_ :: (MonoFoldable mono, Applicative f) => mono -> (Element mono -> f b) -> f ()
     ofor_ = flip otraverse_
-    
+
     omapM_ :: (MonoFoldable mono, Monad m) => (Element mono -> m b) -> mono -> m ()
     omapM_ f = ofoldr ((>>) . f) (return ())
-    
+
     oforM_ :: (MonoFoldable mono, Monad m) => mono -> (Element mono -> m b) -> m ()
     oforM_ = flip omapM_
     {-# INLINE oforM_ #-}
-    
+
     ofoldlM :: (MonoFoldable mono, Monad m) => (a -> Element mono -> m a) -> a -> mono -> m a
     ofoldlM f z0 xs = ofoldr f' return xs z0
       where f' x k z = f z x >>= k
@@ -356,7 +356,10 @@
 instance MonoFoldable [a] where
     otoList = id
     {-# INLINE otoList #-}
-instance MonoFoldable (Maybe a)
+instance MonoFoldable (Maybe a) where
+    omapM_ _ Nothing = return ()
+    omapM_ f (Just x) = f x >> return ()
+    {-# INLINE omapM_ #-}
 instance MonoFoldable (Tree a)
 instance MonoFoldable (Seq a) where
     headEx = flip Seq.index 1