diff --git a/Data/Generics/Uniplate/Data.hs b/Data/Generics/Uniplate/Data.hs
--- a/Data/Generics/Uniplate/Data.hs
+++ b/Data/Generics/Uniplate/Data.hs
@@ -14,6 +14,23 @@
     For faster performance (5x faster, but requires writing instances) switch to
     "Data.Generics.Uniplate.Direct". If you get instance conflicts
     when using both @Data@ and @Direct@, switch to "Data.Generics.Uniplate.DataOnly".
+
+    The instances are faster than GHC because they precompute a table of useful information,
+    then use this information when performing the traversals. Sometimes it is not possible
+    to compute the table, in which case this library will perform about the same speed as
+    SYB.
+
+    Setting the environment variable @$UNIPLATE_VERBOSE@ has the following effects:
+
+  * @-1@ - raise a program error every time construction of the table fails
+
+  * @0@ (or unset) - never print any messages or raise any errors
+
+  * @1@ - give a message every time a table is computed
+
+  * @2@ - give a message when table computation fails
+
+    The @$UNIPLATE_VERBOSE@ environment variable must be set before the first call to uniplate.
 -}
 module Data.Generics.Uniplate.Data(
     module Data.Generics.Uniplate.Operations,
diff --git a/Data/Generics/Uniplate/Internal/Data.hs b/Data/Generics/Uniplate/Internal/Data.hs
--- a/Data/Generics/Uniplate/Internal/Data.hs
+++ b/Data/Generics/Uniplate/Internal/Data.hs
@@ -14,6 +14,8 @@
 import Data.List
 import Data.IORef
 import Control.Exception
+import Control.Monad
+import System.Environment(getEnv)
 import qualified Data.IntMap as IntMap; import Data.IntMap(IntMap)
 
 
@@ -41,7 +43,12 @@
 
 #endif
 
+{-# NOINLINE uniplateVerbose #-}
+uniplateVerbose :: Int -- 0 = quiet, 1 = errors only, 2 = everything
+uniplateVerbose = unsafePerformIO $ do
+    fmap read (getEnv "UNIPLATE_VERBOSE") `Control.Exception.catch` \(_ :: SomeException) -> return 0
 
+
 ---------------------------------------------------------------------
 -- HIT TEST
 
@@ -84,13 +91,18 @@
     case lookup2 kfrom kto follow of
         Just ans -> return ans
         Nothing -> do
-            res <- Control.Exception.catch (return $! Just $! insertHitMap from hit) (\(_ :: SomeException) -> return Nothing)
+            res <- Control.Exception.try (return $! insertHitMap from hit)
             (hit,fol) <- return $ case res of
-                Nothing -> (hit, Nothing)
-                Just hit -> (hit, Just $ follower kfrom kto hit)
-            -- -- uncomment these lines to see where type search fails
-            -- if isNothing fol then print ("failure",show (typeOf vfrom),kfrom,kto) else return ()
+                Left _ -> (hit, Nothing)
+                Right hit -> (hit, Just $ follower kfrom kto hit)
 
+            let msg =
+                    "# Uniplate lookup on (" ++ show (typeOf vfrom) ++ "), from (" ++ show kfrom ++ "), to (" ++ show kto ++ "): " ++
+                    either (\(msg::SomeException) -> "FAILURE (" ++ show msg ++ ")") (const "Success") res
+
+            when (uniplateVerbose + maybe 1 (const 0) fol >= 2) $ putStrLn msg
+            when (uniplateVerbose < 0 && isNothing fol) $ error msg
+
             atomicModifyIORef cache $ \(Cache _ follow) -> (Cache hit (insert2 kfrom kto fol follow), ())
             return fol
 
@@ -164,7 +176,11 @@
 sybChildren :: Data a => a -> [DataBox]
 sybChildren x
     | isAlgType dtyp = concatMap f ctrs
-    | isNorepType dtyp = error "sybChildren on NorepType"
+    | isNorepType dtyp = []
+        -- Extensive discussions with Lennart and Roman decided that if something returns NorepType, it really wants to be atomic
+        -- so we should let it be, and pretend it has no children.
+        -- The most common types which say this are Data.Set/Data.Map, and we think that's a bug in their Data instances.
+        -- error $ "Data.Generics.Uniplate.Data: sybChildren on data type which returns NorepType, " ++ show (typeOf x) ++ ", " ++ show dtyp
     | otherwise = []
     where
         f ctr = gmapQ dataBox (asTypeOf (fromConstr ctr) x)
diff --git a/uniplate.cabal b/uniplate.cabal
--- a/uniplate.cabal
+++ b/uniplate.cabal
@@ -1,7 +1,7 @@
 cabal-version:      >= 1.6
 build-type:         Simple
 name:               uniplate
-version:            1.6.1
+version:            1.6.2
 author:             Neil Mitchell <ndmitchell@gmail.com>
 maintainer:         Neil Mitchell <ndmitchell@gmail.com>
 copyright:          Neil Mitchell 2006-2011