diff --git a/CHANGELOG.markdown b/CHANGELOG.markdown
--- a/CHANGELOG.markdown
+++ b/CHANGELOG.markdown
@@ -1,3 +1,8 @@
+4.12.2
+------
+* Incorporated a bug fix for `foldByOf` and `foldMapByOf` to actually let them work on folds.
+* Added a `Plated` instance for `CofreeT`
+
 4.12.1
 ------
 * The `Simple` type alias is now poly-kinded. This lets you use `Simple Field1 s a` and the like in constraints.
diff --git a/lens.cabal b/lens.cabal
--- a/lens.cabal
+++ b/lens.cabal
@@ -1,6 +1,6 @@
 name:          lens
 category:      Data, Lenses, Generics
-version:       4.12.1
+version:       4.12.2
 license:       BSD3
 cabal-version: >= 1.8
 license-file:  LICENSE
@@ -207,7 +207,7 @@
     text                      >= 0.11     && < 1.3,
     transformers              >= 0.2      && < 0.5,
     transformers-compat       >= 0.4      && < 1,
-    unordered-containers      >= 0.2      && < 0.3,
+    unordered-containers      >= 0.2.4    && < 0.3,
     vector                    >= 0.9      && < 0.12,
     void                      >= 0.5      && < 1
 
@@ -382,6 +382,10 @@
   main-is:        doctests.hs
   ghc-options:    -Wall -threaded
   hs-source-dirs: tests
+
+  if flag(trustworthy) && impl(ghc>=7.2)
+    other-extensions: Trustworthy
+    cpp-options: -DTRUSTWORTHY=1
 
   if !flag(test-doctests)
     buildable: False
diff --git a/src/Control/Lens.hs b/src/Control/Lens.hs
--- a/src/Control/Lens.hs
+++ b/src/Control/Lens.hs
@@ -1,4 +1,5 @@
 {-# LANGUAGE CPP #-}
+{-# LANGUAGE Safe #-}
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Control.Lens
diff --git a/src/Control/Lens/Fold.hs b/src/Control/Lens/Fold.hs
--- a/src/Control/Lens/Fold.hs
+++ b/src/Control/Lens/Fold.hs
@@ -4,6 +4,14 @@
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 
+#ifndef MIN_VERSION_profunctors
+#define MIN_VERSION_profunctors(x,y,z) 1
+#endif
+
+#if __GLASGOW_HASKELL__ < 708 || !(MIN_VERSION_profunctors(4,4,0))
+{-# LANGUAGE Trustworthy #-}
+#endif
+
 {-# OPTIONS_GHC -fno-warn-orphans #-}
 ----------------------------------------------------------------------------
 -- |
@@ -160,6 +168,7 @@
 import Data.Profunctor.Rep
 import Data.Profunctor.Sieve
 import Data.Profunctor.Unsafe
+import Data.Reflection
 import Data.Traversable
 import Prelude hiding (foldr)
 
@@ -2487,14 +2496,70 @@
 -- Folds with Reified Monoid
 ------------------------------------------------------------------------------
 
+-- | Fold a value using its 'Foldable' instance using
+-- explicitly provided 'Monoid' operations. This is like 'fold'
+-- where the 'Monoid' instance can be manually specified.
+--
+-- @
+-- 'foldBy' 'mappend' 'mempty' ≡ 'fold'
+-- @
+--
+-- >>> foldBy (++) [] ["hello","world"]
+-- "helloworld"
 foldBy :: Foldable t => (a -> a -> a) -> a -> t a -> a
 foldBy f z = reifyFold f z (foldMap M)
 
-foldByOf :: (forall i. Getting (M a i) s a) -> (a -> a -> a) -> a -> s -> a
+-- | Fold a value using a specified 'Fold' and 'Monoid' operations.
+-- This is like 'foldBy' where the 'Foldable' instance can be
+-- manually specified.
+--
+-- @
+-- 'foldByOf' 'folded' ≡ 'foldBy'
+-- @
+--
+-- @
+-- 'foldByOf' :: 'Getter' s a     -> (a -> a -> a) -> a -> s -> a
+-- 'foldByOf' :: 'Fold' s a       -> (a -> a -> a) -> a -> s -> a
+-- 'foldByOf' :: 'Lens'' s a      -> (a -> a -> a) -> a -> s -> a
+-- 'foldByOf' :: 'Traversal'' s a -> (a -> a -> a) -> a -> s -> a
+-- 'foldByOf' :: 'Iso'' s a       -> (a -> a -> a) -> a -> s -> a
+-- @
+--
+-- >>> foldByOf both (++) [] ("hello","world")
+-- "helloworld"
+foldByOf :: (forall i. Reifies i (ReifiedMonoid a) => Getting (M a i) s a) -> (a -> a -> a) -> a -> s -> a
 foldByOf l f z = reifyFold f z (foldMapOf l M)
 
+-- | Fold a value using its 'Foldable' instance using
+-- explicitly provided 'Monoid' operations. This is like 'foldMap'
+-- where the 'Monoid' instance can be manually specified.
+--
+-- @
+-- 'foldMapBy' 'mappend' 'mempty' ≡ 'foldMap'
+-- @
+--
+-- >>> foldMapBy (+) 0 length ["hello","world"]
+-- 10
 foldMapBy :: Foldable t => (r -> r -> r) -> r -> (a -> r) -> t a -> r
 foldMapBy f z g = reifyFold f z (foldMap (M #. g))
 
-foldMapByOf :: (forall s. Getting (M r s) t a) -> (r -> r -> r) -> r -> (a -> r) -> t -> r
+-- | Fold a value using a specified 'Fold' and 'Monoid' operations.
+-- This is like 'foldMapBy' where the 'Foldable' instance can be
+-- manually specified.
+--
+-- @
+-- 'foldMapByOf' 'folded' ≡ 'foldMapBy'
+-- @
+--
+-- @
+-- 'foldMapByOf' :: 'Getter' s a     -> (r -> r -> r) -> r -> (a -> r) -> s -> r
+-- 'foldMapByOf' :: 'Fold' s a       -> (r -> r -> r) -> r -> (a -> r) -> s -> r
+-- 'foldMapByOf' :: 'Traversal'' s a -> (r -> r -> r) -> r -> (a -> r) -> s -> r
+-- 'foldMapByOf' :: 'Lens'' s a      -> (r -> r -> r) -> r -> (a -> r) -> s -> r
+-- 'foldMapByOf' :: 'Iso'' s a       -> (r -> r -> r) -> r -> (a -> r) -> s -> r
+-- @
+--
+-- >>> foldMapByOf both (+) 0 length ("hello","world")
+-- 10
+foldMapByOf :: (forall i. Reifies i (ReifiedMonoid r) => Getting (M r i) s a) -> (r -> r -> r) -> r -> (a -> r) -> s -> r
 foldMapByOf l f z g = reifyFold f z (foldMapOf l (M #. g))
diff --git a/src/Control/Lens/Getter.hs b/src/Control/Lens/Getter.hs
--- a/src/Control/Lens/Getter.hs
+++ b/src/Control/Lens/Getter.hs
@@ -2,6 +2,16 @@
 {-# LANGUAGE Rank2Types #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FlexibleInstances #-}
+
+
+#ifndef MIN_VERSION_profunctors
+#define MIN_VERSION_profunctors(x,y,z) 1
+#endif
+
+#if __GLASGOW_HASKELL__ < 708 || !(MIN_VERSION_profunctors(4,4,0))
+{-# LANGUAGE Trustworthy #-}
+#endif
+
 #if __GLASGOW_HASKELL__ && __GLASGOW_HASKELL__ >= 704
 {-# LANGUAGE NoPolyKinds #-}
 {-# LANGUAGE NoDataKinds #-}
diff --git a/src/Control/Lens/Internal/Indexed.hs b/src/Control/Lens/Internal/Indexed.hs
--- a/src/Control/Lens/Internal/Indexed.hs
+++ b/src/Control/Lens/Internal/Indexed.hs
@@ -5,6 +5,15 @@
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
+
+#ifndef MIN_VERSION_profunctors
+#define MIN_VERSION_profunctors(x,y,z) 1
+#endif
+
+#if __GLASGOW_HASKELL__ < 708 || !(MIN_VERSION_profunctors(4,4,0))
+{-# LANGUAGE Trustworthy #-}
+#endif
+
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Control.Lens.Internal.Indexed
diff --git a/src/Control/Lens/Internal/Setter.hs b/src/Control/Lens/Internal/Setter.hs
--- a/src/Control/Lens/Internal/Setter.hs
+++ b/src/Control/Lens/Internal/Setter.hs
@@ -1,4 +1,13 @@
 {-# LANGUAGE CPP #-}
+
+#ifndef MIN_VERSION_profunctors
+#define MIN_VERSION_profunctors(x,y,z) 1
+#endif
+
+#if __GLASGOW_HASKELL__ < 708 || !(MIN_VERSION_profunctors(4,4,0))
+{-# LANGUAGE Trustworthy #-}
+#endif
+
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Control.Lens.Internal.Setter
diff --git a/src/Control/Lens/Iso.hs b/src/Control/Lens/Iso.hs
--- a/src/Control/Lens/Iso.hs
+++ b/src/Control/Lens/Iso.hs
@@ -6,6 +6,14 @@
 #ifndef MIN_VERSION_bytestring
 #define MIN_VERSION_bytestring(x,y,z) 1
 #endif
+
+#ifndef MIN_VERSION_profunctors
+#define MIN_VERSION_profunctors(x,y,z) 1
+#endif
+
+#if __GLASGOW_HASKELL__ < 708 || !(MIN_VERSION_profunctors(4,4,0))
+{-# LANGUAGE Trustworthy #-}
+#endif
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Control.Lens.Iso
diff --git a/src/Control/Lens/Lens.hs b/src/Control/Lens/Lens.hs
--- a/src/Control/Lens/Lens.hs
+++ b/src/Control/Lens/Lens.hs
@@ -9,6 +9,14 @@
 #ifndef MIN_VERSION_mtl
 #define MIN_VERSION_mtl(x,y,z) 1
 #endif
+
+#ifndef MIN_VERSION_profunctors
+#define MIN_VERSION_profunctors(x,y,z) 1
+#endif
+
+#if __GLASGOW_HASKELL__ < 708 || !(MIN_VERSION_profunctors(4,4,0))
+{-# LANGUAGE Trustworthy #-}
+#endif
 -------------------------------------------------------------------------------
 -- |
 -- Module      :  Control.Lens.Lens
diff --git a/src/Control/Lens/Level.hs b/src/Control/Lens/Level.hs
--- a/src/Control/Lens/Level.hs
+++ b/src/Control/Lens/Level.hs
@@ -1,6 +1,14 @@
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE Rank2Types #-}
 {-# LANGUAGE FlexibleContexts #-}
+
+#ifndef MIN_VERSION_profunctors
+#define MIN_VERSION_profunctors(x,y,z) 1
+#endif
+
+#if __GLASGOW_HASKELL__ < 708 || !(MIN_VERSION_profunctors(4,4,0))
+{-# LANGUAGE Trustworthy #-}
+#endif
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Control.Lens.Level
diff --git a/src/Control/Lens/Plated.hs b/src/Control/Lens/Plated.hs
--- a/src/Control/Lens/Plated.hs
+++ b/src/Control/Lens/Plated.hs
@@ -98,6 +98,7 @@
 
 import Control.Applicative
 import Control.Comonad.Cofree
+import qualified Control.Comonad.Trans.Cofree as CoTrans
 import Control.Lens.Fold
 import Control.Lens.Getter
 import Control.Lens.Indexed
@@ -112,7 +113,6 @@
 import Control.MonadPlus.Free as MonadPlus
 #endif
 import qualified Language.Haskell.TH as TH
-import Data.Bitraversable
 import Data.Data
 import Data.Data.Lens
 import Data.Monoid
@@ -232,7 +232,7 @@
   plate _ x         = pure x
 
 instance (Traversable f, Traversable m) => Plated (Trans.FreeT f m a) where
-  plate f (Trans.FreeT xs) = Trans.FreeT <$> traverse (bitraverse pure f) xs
+  plate f (Trans.FreeT xs) = Trans.FreeT <$> traverse (traverse f) xs
 
 #if !(MIN_VERSION_free(4,6,0))
 instance Traversable f => Plated (MonadPlus.Free f a) where
@@ -248,6 +248,9 @@
 --
 -- instance (Traversable f, Traversable m) => Plated (ChurchT.FT f m a) where
 --   plate f = fmap ChurchT.toFT . plate (fmap ChurchT.fromFT . f . ChurchT.toFT) . ChurchT.fromFT
+
+instance (Traversable f, Traversable w) => Plated (CoTrans.CofreeT f w a) where
+  plate f (CoTrans.CofreeT xs) = CoTrans.CofreeT <$> traverse (traverse f) xs
 
 instance Traversable f => Plated (Cofree f a) where
   plate f (a :< as) = (:<) a <$> traverse f as
diff --git a/src/Control/Lens/Review.hs b/src/Control/Lens/Review.hs
--- a/src/Control/Lens/Review.hs
+++ b/src/Control/Lens/Review.hs
@@ -1,5 +1,13 @@
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE Rank2Types #-}
+
+#ifndef MIN_VERSION_profunctors
+#define MIN_VERSION_profunctors(x,y,z) 1
+#endif
+
+#if __GLASGOW_HASKELL__ < 708 || !(MIN_VERSION_profunctors(4,4,0))
+{-# LANGUAGE Trustworthy #-}
+#endif
 -------------------------------------------------------------------------------
 -- |
 -- Module      :  Control.Lens.Review
diff --git a/src/Control/Lens/Setter.hs b/src/Control/Lens/Setter.hs
--- a/src/Control/Lens/Setter.hs
+++ b/src/Control/Lens/Setter.hs
@@ -3,6 +3,14 @@
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE FlexibleContexts #-}
+
+#ifndef MIN_VERSION_profunctors
+#define MIN_VERSION_profunctors(x,y,z) 1
+#endif
+
+#if __GLASGOW_HASKELL__ < 708 || !(MIN_VERSION_profunctors(4,4,0))
+{-# LANGUAGE Trustworthy #-}
+#endif
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Control.Lens.Setter
diff --git a/src/Control/Lens/Zoom.hs b/src/Control/Lens/Zoom.hs
--- a/src/Control/Lens/Zoom.hs
+++ b/src/Control/Lens/Zoom.hs
@@ -10,6 +10,14 @@
 #define MIN_VERSION_mtl(x,y,z) 1
 #endif
 
+#ifndef MIN_VERSION_profunctors
+#define MIN_VERSION_profunctors(x,y,z) 1
+#endif
+
+#if __GLASGOW_HASKELL__ < 708 || !(MIN_VERSION_profunctors(4,4,0))
+{-# LANGUAGE Trustworthy #-}
+#endif
+
 -------------------------------------------------------------------------------
 -- |
 -- Module      :  Control.Lens.Zoom
diff --git a/tests/doctests.hsc b/tests/doctests.hsc
--- a/tests/doctests.hsc
+++ b/tests/doctests.hsc
@@ -60,6 +60,9 @@
   : "-optP-include"
   : "-optPdist/build/autogen/cabal_macros.h"
   : "-hide-all-packages"
+#ifdef TRUSTWORTHY
+  : "-DTRUSTWORTHY=1"
+#endif
   : map ("-package="++) deps ++ sources
 
 getSources :: IO [FilePath]