diff --git a/src/Data/Validity.hs b/src/Data/Validity.hs
--- a/src/Data/Validity.hs
+++ b/src/Data/Validity.hs
@@ -461,12 +461,19 @@
 #endif
 -- | Valid if the contained numbers are valid and the denominator is
 -- strictly positive.
-instance (Num a, Ord a, Validity a) => Validity (Ratio a) where
+instance (Validity a, Ord a, Num a, Integral a) => Validity (Ratio a) where
     validate (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
             ]
 
 -- | Valid according to the contained 'Integer'.
diff --git a/validity.cabal b/validity.cabal
--- a/validity.cabal
+++ b/validity.cabal
@@ -1,11 +1,13 @@
--- This file has been generated from package.yaml by hpack version 0.28.2.
+cabal-version: 1.12
+
+-- This file has been generated from package.yaml by hpack version 0.31.1.
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: d7dbe113de14753bb19dacab5a6fad8e6ac1fd75f7d6157d10c29c478a6b267f
+-- hash: ea622d10d177b4be018cdfc126d397fdf597d052e886eb69bc3a7d334f8bcfca
 
 name:           validity
-version:        0.9.0.0
+version:        0.9.0.1
 synopsis:       Validity typeclass
 description:    For more info, see <https://github.com/NorfairKing/validity the readme>.
                 .
@@ -40,7 +42,6 @@
 license:        MIT
 license-file:   LICENSE
 build-type:     Simple
-cabal-version:  >= 1.10
 
 source-repository head
   type: git