diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,6 +1,12 @@
 # ChangeLog / ReleaseNotes
 
 
+## Version 0.2.1.0
+
+* Depend on `quickcheck-instances` to provide `instance Arbitrary Natural` for
+  unit tests. (**minor change**)
+
+
 ## Version 0.2.0.0
 
 * Instances for `Integer` and `Natural`.
diff --git a/number-length.cabal b/number-length.cabal
--- a/number-length.cabal
+++ b/number-length.cabal
@@ -1,5 +1,5 @@
 name:                   number-length
-version:                0.2.0.0
+version:                0.2.1.0
 synopsis:
   Number of digits in a number in decimal and hexadecimal representation.
 description:
@@ -102,6 +102,9 @@
     other-modules:
         TestCase.Data.NumberLength.Natural
       , Data.NumberLength.Natural
+    build-depends:
+        quickcheck-instances >=0.3.15 && <1
+        -- ^ Version 0.3.15 is the first that contains instances for Natural.
 
   build-depends:
       base >=4.5 && <5
@@ -184,4 +187,4 @@
 source-repository this
   type:                 git
   location:             git://github.com/trskop/number-length.git
-  tag:                  v0.2.0.0
+  tag:                  v0.2.1.0
diff --git a/test/TestCase/Data/NumberLength.hs b/test/TestCase/Data/NumberLength.hs
--- a/test/TestCase/Data/NumberLength.hs
+++ b/test/TestCase/Data/NumberLength.hs
@@ -3,7 +3,7 @@
 -- |
 -- Module:       $HEADER$
 -- Description:  TODO
--- Copyright:    (c) 2015-2016, Peter Trško
+-- Copyright:    (c) 2015-2018, Peter Trško
 -- License:      BSD3
 --
 -- Stability:    experimental
@@ -29,6 +29,9 @@
 import Test.Framework.Providers.HUnit (testCase)
 import Test.Framework.Providers.QuickCheck2 (testProperty)
 import Test.HUnit ((@?=))
+#ifdef HAVE_NATURAL
+import Test.QuickCheck.Instances ()
+#endif
 
 import Data.NumberLength
     ( BoundedNumberLength(maxNumberLength, maxNumberLengthHex)
diff --git a/test/TestCase/Data/NumberLength/Int.hs b/test/TestCase/Data/NumberLength/Int.hs
--- a/test/TestCase/Data/NumberLength/Int.hs
+++ b/test/TestCase/Data/NumberLength/Int.hs
@@ -2,7 +2,7 @@
 -- |
 -- Module:       $HEADER$
 -- Description:  TODO
--- Copyright:    (c) 2015-2016, Peter Trško
+-- Copyright:    (c) 2015-2018, Peter Trško
 -- License:      BSD3
 --
 -- Stability:    experimental
@@ -176,6 +176,7 @@
 
 property_lengthInt :: Int -> Bool
 property_lengthInt = lengthInt <==> numberLengthDec
+{-# ANN property_lengthInt "HLint: ignore Use camelCase" #-}
 
 -- }}} lengthInt --------------------------------------------------------------
 -- }}} Decimal ----------------------------------------------------------------
@@ -252,6 +253,7 @@
 
 property_lengthIntHex :: Int -> Bool
 property_lengthIntHex = lengthIntHex <==> numberLengthHex
+{-# ANN property_lengthIntHex "HLint: ignore Use camelCase" #-}
 
 -- }}} lengthIntHex -----------------------------------------------------------
 -- }}} Hexadecimal ------------------------------------------------------------
diff --git a/test/TestCase/Data/NumberLength/Integer.hs b/test/TestCase/Data/NumberLength/Integer.hs
--- a/test/TestCase/Data/NumberLength/Integer.hs
+++ b/test/TestCase/Data/NumberLength/Integer.hs
@@ -2,7 +2,7 @@
 -- |
 -- Module:       $HEADER$
 -- Description:  TODO
--- Copyright:    (c) 2015-2016, Peter Trško
+-- Copyright:    (c) 2015-2018, Peter Trško
 -- License:      BSD3
 --
 -- Stability:    experimental
@@ -95,8 +95,10 @@
 test_lengthInteger n = lengthInteger n @?= numberLengthDec n
 test_lengthIntegerHex n = lengthIntegerHex n @?= numberLengthHex n
 
-property_lengthInteger :: Integer -> Bool
+property_lengthInteger, property_lengthIntegerHex :: Integer -> Bool
+
 property_lengthInteger = lengthInteger <==> numberLengthDec
+{-# ANN property_lengthInteger "HLint: ignore Use camelCase" #-}
 
-property_lengthIntegerHex :: Integer -> Bool
 property_lengthIntegerHex = lengthIntegerHex <==> numberLengthHex
+{-# ANN property_lengthIntegerHex "HLint: ignore Use camelCase" #-}
diff --git a/test/TestCase/Data/NumberLength/Natural.hs b/test/TestCase/Data/NumberLength/Natural.hs
--- a/test/TestCase/Data/NumberLength/Natural.hs
+++ b/test/TestCase/Data/NumberLength/Natural.hs
@@ -2,7 +2,7 @@
 -- |
 -- Module:       $HEADER$
 -- Description:  TODO
--- Copyright:    (c) 2015-2016, Peter Trško
+-- Copyright:    (c) 2015-2018, Peter Trško
 -- License:      BSD3
 --
 -- Stability:    experimental
@@ -33,6 +33,7 @@
 import Test.Framework.Providers.HUnit (testCase)
 import Test.Framework.Providers.QuickCheck2 (testProperty)
 import Test.HUnit (Assertion, (@?=))
+import Test.QuickCheck.Instances ()
 
 import Data.NumberLength.Natural
     ( lengthNatural
@@ -84,5 +85,9 @@
 test_lengthNaturalHex n = lengthNaturalHex n @?= numberLengthHex n
 
 property_lengthNatural, property_lengthNaturalHex :: Natural -> Bool
+
 property_lengthNatural = lengthNatural <==> numberLengthDec
+{-# ANN property_lengthNatural "HLint: ignore Use camelCase" #-}
+
 property_lengthNaturalHex = lengthNaturalHex <==> numberLengthHex
+{-# ANN property_lengthNaturalHex "HLint: ignore Use camelCase" #-}
diff --git a/test/TestCase/Data/NumberLength/Word.hs b/test/TestCase/Data/NumberLength/Word.hs
--- a/test/TestCase/Data/NumberLength/Word.hs
+++ b/test/TestCase/Data/NumberLength/Word.hs
@@ -2,7 +2,7 @@
 -- |
 -- Module:       $HEADER$
 -- Description:  TODO
--- Copyright:    (c) 2015-2016, Peter Trško
+-- Copyright:    (c) 2015-2018, Peter Trško
 -- License:      BSD3
 --
 -- Stability:    experimental
@@ -164,6 +164,7 @@
 
 property_lengthWord :: Word -> Bool
 property_lengthWord = lengthWord <==> numberLength
+{-# ANN property_lengthWord "HLint: ignore Use camelCase" #-}
 
 -- }}} lengthWord -------------------------------------------------------------
 -- }}} Decimal ----------------------------------------------------------------
@@ -237,6 +238,7 @@
 
 property_lengthWordHex :: Word -> Bool
 property_lengthWordHex = lengthWordHex <==> numberLengthHex
+{-# ANN property_lengthWordHex "HLint: ignore Use camelCase" #-}
 
 -- }}} lengthWordHex ----------------------------------------------------------
 -- }}} Hexadecimal ------------------------------------------------------------