diff --git a/changelog b/changelog
--- a/changelog
+++ b/changelog
@@ -1,6 +1,6 @@
 -*-change-log-*-
 
-0.1.6.0 Ross Paterson <R.Paterson@city.ac.uk> May 2025
+0.1.6.1 Ross Paterson <R.Paterson@city.ac.uk> May 2025
 	* Added NFData instances
 
 0.1.5.0 Ross Paterson <R.Paterson@city.ac.uk> Jan 2022
diff --git a/fingertree.cabal b/fingertree.cabal
--- a/fingertree.cabal
+++ b/fingertree.cabal
@@ -1,5 +1,5 @@
 Name:           fingertree
-Version:        0.1.6.0
+Version:        0.1.6.1
 Cabal-Version:  1.18
 Copyright:      (c) 2006 Ross Paterson, Ralf Hinze
 License:        BSD3
@@ -34,7 +34,7 @@
 Library
   Build-Depends:
                 base < 6,
-                deepseq >= 1.4 && < 1.5
+                deepseq >= 1.3 && < 1.6
   Default-Language: Haskell2010
   Other-Extensions:
                 MultiParamTypeClasses
@@ -53,7 +53,7 @@
   default-language: Haskell2010
   build-depends:
                 base >= 4.2 && < 6,
-                deepseq >= 1.4 && < 1.5,
+                deepseq >= 1.3 && < 1.6,
                 HUnit,
                 QuickCheck,
                 test-framework,
diff --git a/tests/ft-properties.hs b/tests/ft-properties.hs
--- a/tests/ft-properties.hs
+++ b/tests/ft-properties.hs
@@ -479,14 +479,14 @@
 evalM m = snd (runM m 0)
 
 instance Monad M where
-    return x = M $ \ n -> (n, x)
+    return = pure
     M u >>= f = M $ \ m -> let (n, x) = u m in runM (f x) n
 
 instance Functor M where
     fmap f (M u) = M $ \ m -> let (n, x) = u m in (n, f x)
 
 instance Applicative M where
-    pure = return
+    pure x = M $ \ n -> (n, x)
     (<*>) = ap
 
 step :: M Int