diff --git a/src/Data/Validity.hs b/src/Data/Validity.hs
--- a/src/Data/Validity.hs
+++ b/src/Data/Validity.hs
@@ -1,8 +1,8 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE DefaultSignatures #-}
-{-# OPTIONS_GHC -Wno-redundant-constraints #-}
 
 {-|
 
@@ -43,14 +43,38 @@
 
 import Data.Fixed (Fixed(MkFixed), HasResolution)
 import Data.Maybe (Maybe, fromMaybe)
+import Data.Word (Word, Word8, Word16)
 import GHC.Generics
+#if MIN_VERSION_base(4,8,0)
 import GHC.Natural (Natural, isValidNatural)
+#endif
 import GHC.Real (Ratio(..))
 
 -- | A class of types that have additional invariants defined upon them
 -- that aren't enforced by the type system
+--
+-- 'isValid' should be an underapproximation of actual validity.
+--
+-- This means that if 'isValid' is not a perfect representation of actual
+-- validity, for safety reasons, it should never return 'True' for invalid
+-- values, but it may return 'False' for valid values.
+--
+-- For example:
+--
+-- > isValid = const False
+--
+-- is a valid implementation for any type, because it never returns 'True'
+-- for invalid values.
+--
+-- > isValid (Even i) = i == 2
+--
+-- is a valid implementation for @newtype Even = Even Int@, but
+--
+-- > isValid (Even i) = even i || i == 1
+--
+-- is not because it returns 'True' for an invalid value: '1'.
 class Validity a where
-    isValid :: a -> Bool -- ^ Check whether a given value is a valid value.
+    isValid :: a -> Bool
     default isValid :: (Generic a, GValidity (Rep a)) =>
         a -> Bool
     isValid = gIsValid . from
@@ -118,6 +142,14 @@
 instance Validity Word where
     isValid = const True
 
+-- | Trivially valid
+instance Validity Word8 where
+    isValid = const True
+
+-- | Trivially valid
+instance Validity Word16 where
+    isValid = const True
+
 -- | NOT trivially valid:
 --
 -- * NaN is not valid.
@@ -142,11 +174,13 @@
 -- Even though this is not technically sound, it is good enough for now.
 instance Validity Integer where
     isValid = const True
-
+#if MIN_VERSION_base(4,8,0)
 -- | Valid according to 'isValidNatural'
+--
+-- Only available with @base >= 4.8@.
 instance Validity Natural where
     isValid = isValidNatural
-
+#endif
 -- | Valid if the contained 'Integer's are valid and the denominator is
 -- strictly positive.
 instance Validity Rational where
diff --git a/validity.cabal b/validity.cabal
--- a/validity.cabal
+++ b/validity.cabal
@@ -1,5 +1,5 @@
 name: validity
-version: 0.3.2.0
+version: 0.3.3.0
 cabal-version: >=1.10
 build-type: Simple
 license: MIT
@@ -34,7 +34,8 @@
         Data.RelativeValidity
         Data.Validity
     build-depends:
-        base >=4.8 && <5
+        base >=4.7 && <5
     default-language: Haskell2010
     hs-source-dirs: src
-
+    if impl(ghc >= 8.0.0)
+        ghc-options: -Wno-redundant-constraints