diff --git a/Control/Concurrent/STM/TArray.hs b/Control/Concurrent/STM/TArray.hs
--- a/Control/Concurrent/STM/TArray.hs
+++ b/Control/Concurrent/STM/TArray.hs
@@ -4,9 +4,11 @@
 {-# LANGUAGE Trustworthy #-}
 #endif
 
-#define HAS_UNLIFTED_ARRAY defined(__GLASGOW_HASKELL__) && __GLASGOW_HASKELL__ >= 904
+#if defined(__GLASGOW_HASKELL__) && __GLASGOW_HASKELL__ >= 904
+#define HAS_UNLIFTED_ARRAY 1
+#endif
 
-#if HAS_UNLIFTED_ARRAY
+#if defined(HAS_UNLIFTED_ARRAY)
 {-# LANGUAGE MagicHash, UnboxedTuples #-}
 #endif
 
@@ -30,7 +32,7 @@
 
 import Control.Monad.STM (STM, atomically)
 import Data.Typeable (Typeable)
-#if HAS_UNLIFTED_ARRAY
+#if defined(HAS_UNLIFTED_ARRAY)
 import Control.Concurrent.STM.TVar (readTVar, readTVarIO, writeTVar)
 import Data.Array.Base (safeRangeSize, MArray(..))
 import Data.Ix (Ix)
@@ -47,7 +49,7 @@
 -- interface for mutable arrays.
 --
 -- It is conceptually implemented as @Array i (TVar e)@.
-#if HAS_UNLIFTED_ARRAY
+#if defined(HAS_UNLIFTED_ARRAY)
 data TArray i e = TArray
     !i   -- lower bound
     !i   -- upper bound
diff --git a/changelog.md b/changelog.md
--- a/changelog.md
+++ b/changelog.md
@@ -1,5 +1,9 @@
 # Changelog for [`stm` package](http://hackage.haskell.org/package/stm)
 
+## 2.5.2.1 *September 2023*
+
+  * Eliminate reliance on undefined CPP behavior ([#75](https://github.com/haskell/stm/issues/75))
+
 ## 2.5.2.0 *September 2023*
 
   * Fix strictness of `stateTVar` ([#30](https://github.com/haskell/stm/ssues/30))
diff --git a/stm.cabal b/stm.cabal
--- a/stm.cabal
+++ b/stm.cabal
@@ -1,6 +1,6 @@
 cabal-version:  >=1.10
 name:           stm
-version:        2.5.2.0
+version:        2.5.2.1
 -- don't forget to update changelog.md file!
 
 license:        BSD3