diff --git a/LICENSE b/LICENSE
--- a/LICENSE
+++ b/LICENSE
@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2016 Tom Sydney Kerckhove
+Copyright (c) 2016-2020 Tom Sydney Kerckhove
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
diff --git a/src/Data/Validity.hs b/src/Data/Validity.hs
--- a/src/Data/Validity.hs
+++ b/src/Data/Validity.hs
@@ -95,7 +95,7 @@
 import Data.Ratio
 #endif
 import Data.Bits ((.&.))
-import Data.Char (Char, ord)
+import Data.Char (ord)
 import Data.Int (Int16, Int32, Int64, Int8)
 #if MIN_VERSION_base(4,8,0)
 import Data.Word (Word16, Word32, Word64, Word8)
@@ -106,7 +106,7 @@
 #if MIN_VERSION_base(4,8,0)
 import GHC.Natural
 #endif
-import GHC.Real (Ratio(..), reduce)
+import GHC.Real (Ratio(..))
 
 -- | A class of types that have additional invariants defined upon them
 
@@ -474,7 +474,15 @@
     _ -> True
 
 validateRatioNormalised :: Integral a => Ratio a -> Validation
-validateRatioNormalised r@(n :% d) = declare "The Ratio is normalised." $ reduce n d == r
+validateRatioNormalised (n :% d) = declare "The Ratio is normalised." $
+  case d of
+    0 -> False
+    _ ->
+      let g = gcd n d
+          gcdOverflows = g < 0
+          n' :% d' = (n `quot` g) :% (d `quot` g)
+          valueIsNormalised = n' :% d' == n :% d
+      in not gcdOverflows && valueIsNormalised
 
 -- | Trivially valid
 --
@@ -496,18 +504,12 @@
 -- | Valid if the contained numbers are valid and the denominator is
 -- strictly positive.
 instance (Validity a, Ord a, Num a, Integral a) => Validity (Ratio a) where
-    validate (n :% d) =
+    validate r@(n :% d) =
         mconcat
             [ annotate n "The numerator"
             , annotate d "The denominator"
             , declare "The denominator is strictly positive." $ d > 0
-            , declare "The ratio is normalised" $
-                case d of
-                  0 -> False
-                  _ ->
-                    let g = gcd n d
-                        n' :% d' = (n `quot` g) :% (d `quot` g)
-                    in n' :% d' == n :% d
+            , validateRatioNormalised r
             ]
 
 -- | Valid according to the contained 'Integer'.
diff --git a/test/Data/ValiditySpec.hs b/test/Data/ValiditySpec.hs
--- a/test/Data/ValiditySpec.hs
+++ b/test/Data/ValiditySpec.hs
@@ -56,6 +56,15 @@
         prettyValidation (validateCharNotUtf16SurrogateCodePoint 'a') `shouldSatisfy` isNothing
       it "Says that \\55810 is an invalid char" $
         prettyValidation (validateCharNotUtf16SurrogateCodePoint '\55810') `shouldSatisfy` isJust
+  describe "Ratio" $ do
+    it "says that 0 is valid" $ NormalisedRatio (0 :% 1 :: Ratio Int) `shouldSatisfy` isValid
+    it "says that 1 is valid" $ NormalisedRatio (1 :% 1 :: Ratio Int) `shouldSatisfy` isValid
+    it "says that minBound is valid" $ NormalisedRatio (minBound :% 1 :: Ratio Int) `shouldSatisfy` isValid
+    it "says that maxBound is valid" $ NormalisedRatio (maxBound :% 1 :: Ratio Int) `shouldSatisfy` isValid
+    it "says that maxBound / minBound is invalid" $ NormalisedRatio (maxBound :% minBound :: Ratio Int) `shouldSatisfy` (not . isValid)
+    it "says that minBound / maxBound is invalid" $ NormalisedRatio (minBound :% maxBound :: Ratio Int) `shouldSatisfy` (not . isValid)
+
+    it "says that minBound / 2957808295740799111 is valid" $ NormalisedRatio (minBound :% (2957808295740799111) :: Ratio Int) `shouldSatisfy` isValid
   describe "NormalisedRatio" $ do
     it "says that NaN is invalid" $ NormalisedRatio notANumber `shouldSatisfy` (not . isValid)
     it "says that +Inf is invalid" $ NormalisedRatio infinity `shouldSatisfy` (not . isValid)
diff --git a/validity.cabal b/validity.cabal
--- a/validity.cabal
+++ b/validity.cabal
@@ -1,13 +1,13 @@
 cabal-version: 1.12
 
--- This file has been generated from package.yaml by hpack version 0.31.2.
+-- This file has been generated from package.yaml by hpack version 0.33.0.
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: c5fd8a29bca818acb5b12b926b6d1143f48fcf0d4dbe5ab886a92c7d0492ab08
+-- hash: d006c09ce2c1b3a5b9614851314390ed87a2dd35f3304de883115ca80df6a853
 
 name:           validity
-version:        0.9.0.2
+version:        0.9.0.3
 synopsis:       Validity typeclass
 description:    For more info, see <https://github.com/NorfairKing/validity the readme>.
                 .
@@ -36,9 +36,8 @@
 homepage:       https://github.com/NorfairKing/validity#readme
 bug-reports:    https://github.com/NorfairKing/validity/issues
 author:         Tom Sydney Kerckhove
-maintainer:     syd.kerckhove@gmail.com,
-                nick.van.den.broeck666@gmail.com
-copyright:      Copyright: (c) 2016-2018 Tom Sydney Kerckhove
+maintainer:     syd@cs-syd.eu
+copyright:      Copyright: (c) 2016-2020 Tom Sydney Kerckhove
 license:        MIT
 license-file:   LICENSE
 build-type:     Simple
@@ -71,7 +70,7 @@
       test
   ghc-options: -threaded -rtsopts -with-rtsopts=-N -Wall
   build-depends:
-      base
+      base >=4.7 && <5
     , hspec
     , validity
   default-language: Haskell2010