diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,5 +1,11 @@
 # ChangeLog
 
+## 0.5.1.1
+
+* Update to the instances for generics, to improve error messages for
+  sum types with more than 255 constructors.  See
+  [#141](https://github.com/fpco/store/issues/141)
+
 ## 0.5.1.0
 
 * Update to TH to support sum types with more than 62 constructors.
diff --git a/src/Data/Store/Impl.hs b/src/Data/Store/Impl.hs
--- a/src/Data/Store/Impl.hs
+++ b/src/Data/Store/Impl.hs
@@ -1,4 +1,6 @@
 {-# LANGUAGE BangPatterns #-}
+{-# LANGUAGE ConstraintKinds #-}
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE DefaultSignatures #-}
 {-# LANGUAGE DeriveDataTypeable #-}
@@ -28,6 +30,7 @@
 import           Data.Typeable (Typeable, typeRep)
 import           Data.Word
 import           Foreign.Storable (Storable, sizeOf)
+import           GHC.Exts (Constraint)
 import           GHC.Generics
 import           GHC.TypeLits
 import           Prelude
@@ -262,20 +265,36 @@
 -- FIXME: check that this type level stuff dosen't get turned into
 -- costly runtime computation
 
-instance (SumArity (a :+: b) <= 255, GStoreSizeSum 0 (a :+: b))
+instance (FitsInByte (SumArity (a :+: b)), GStoreSizeSum 0 (a :+: b))
          => GStoreSize (a :+: b) where
     gsize = VarSize $ \x -> sizeOf (undefined :: Word8) + gsizeSum x (Proxy :: Proxy 0)
     {-# INLINE gsize #-}
-instance (SumArity (a :+: b) <= 255, GStorePokeSum 0 (a :+: b))
+instance (FitsInByte (SumArity (a :+: b)), GStorePokeSum 0 (a :+: b))
          => GStorePoke (a :+: b) where
     gpoke x = gpokeSum x (Proxy :: Proxy 0)
     {-# INLINE gpoke #-}
-instance (SumArity (a :+: b) <= 255, GStorePeekSum 0 (a :+: b))
+instance (FitsInByte (SumArity (a :+: b)), GStorePeekSum 0 (a :+: b))
          => GStorePeek (a :+: b) where
     gpeek = do
         tag <- peekStorable
         gpeekSum tag (Proxy :: Proxy 0)
     {-# INLINE gpeek #-}
+
+-- See https://github.com/fpco/store/issues/141 - this constraint type
+-- family machinery improves error messages for generic deriving on
+-- sum types with many constructors.
+
+type FitsInByte n = FitsInByteResult (n <=? 255)
+
+type family FitsInByteResult (b :: Bool) :: Constraint where
+    FitsInByteResult True = ()
+    FitsInByteResult False = TypeErrorMessage
+        "Generic deriving of Store instances can only be used on datatypes with fewer than 256 constructors."
+
+type family TypeErrorMessage (a :: Symbol) :: Constraint where
+#if MIN_VERSION_base(4,9,0)
+    TypeErrorMessage a = TypeError (Text a)
+#endif
 
 -- Similarly to splitting up the generic class into multiple classes, we
 -- also split up the one for sum types.
diff --git a/store.cabal b/store.cabal
--- a/store.cabal
+++ b/store.cabal
@@ -4,10 +4,10 @@
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 5c038df3338fc7661664ddc082dde9288b48c1153a7680a63b2aed528590535e
+-- hash: a18e85521a544ee521dc7788e9b2194c7c3ec0efbeb6092e401782d962ea22d3
 
 name:           store
-version:        0.5.1.0
+version:        0.5.1.1
 synopsis:       Fast binary serialization
 category:       Serialization, Data
 homepage:       https://github.com/fpco/store#readme