diff --git a/CHANGELOG b/CHANGELOG
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,5 +1,9 @@
 # Changelog
 
+- 0.5.2 (2025-12-28)
+  * Adds an explicit 64-bit architecture constraint.
+  * Tests on GHC.10.3.
+
 - 0.5.1 (2025-12-27)
   * Improves the constant-time semantics in wNAF scalar multiplication,
     replacing "indexing via a constant-time selected value" by performing
diff --git a/lib/Crypto/Curve/Secp256k1.hs b/lib/Crypto/Curve/Secp256k1.hs
--- a/lib/Crypto/Curve/Secp256k1.hs
+++ b/lib/Crypto/Curve/Secp256k1.hs
@@ -1,5 +1,6 @@
 {-# OPTIONS_HADDOCK prune #-}
 {-# LANGUAGE BangPatterns #-}
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE DeriveGeneric #-}
 {-# LANGUAGE DerivingStrategies #-}
 {-# LANGUAGE LambdaCase #-}
@@ -10,6 +11,11 @@
 {-# LANGUAGE UnboxedTuples #-}
 {-# LANGUAGE ViewPatterns #-}
 
+#include "MachDeps.h"
+#if WORD_SIZE_IN_BITS != 64
+#error "ppad-secp256k1 requires a 64-bit architecture"
+#endif
+
 -- |
 -- Module: Crypto.Curve.Secp256k1
 -- Copyright: (c) 2024 Jared Tobin
@@ -137,8 +143,7 @@
 pattern Z = (# Limb 0##, Limb 0##, Limb 0##, Limb 0## #)
 
 pattern P :: Limb4 -> Limb4 -> Limb4 -> Projective
-pattern P x y z =
-  Projective (C.Montgomery x) (C.Montgomery y) (C.Montgomery z)
+pattern P x y z = Projective (C.Montgomery x) (C.Montgomery y) (C.Montgomery z)
 {-# COMPLETE P #-}
 
 -- utilities ------------------------------------------------------------------
@@ -285,10 +290,7 @@
 
 -- modQ via conditional subtraction
 modQ :: Wider -> Wider
-modQ x =
-  let !(Wider xw) = x
-      !(Wider qw) = _CURVE_Q
-  in  W.select x (x - _CURVE_Q) (CT.not (W.lt# xw qw))
+modQ x = W.select x (x - _CURVE_Q) (CT.not (W.lt x _CURVE_Q))
 {-# INLINABLE modQ #-}
 
 -- bytewise xor
@@ -442,8 +444,8 @@
 
 -- Constant-time selection of Projective points.
 select_proj :: Projective -> Projective -> CT.Choice -> Projective
-select_proj (P ax ay az) (P bx by bz) c =
-  P (W.select# ax bx c) (W.select# ay by c) (W.select# az bz c)
+select_proj (Projective ax ay az) (Projective bx by bz) c =
+  Projective (C.select ax bx c) (C.select ay by c) (C.select az bz c)
 {-# INLINE select_proj #-}
 
 -- unboxed internals ----------------------------------------------------------
@@ -607,7 +609,7 @@
 
               !abs_b       = CT.select_word# b0 (Exts.minusWord# mnum b0) bor
               !is_zero     = CT.from_word_eq# b0 0##
-              !c0          = CT.from_word# (Exts.and# w 1##)
+              !c0          = CT.from_bit# (Exts.and# w 1##)
               !off_nz      = Exts.minusWord# (Exts.plusWord# off0 abs_b) 1##
               !off         = CT.select_word# off0 off_nz (CT.not is_zero)
 
diff --git a/ppad-secp256k1.cabal b/ppad-secp256k1.cabal
--- a/ppad-secp256k1.cabal
+++ b/ppad-secp256k1.cabal
@@ -1,6 +1,6 @@
 cabal-version:      3.0
 name:               ppad-secp256k1
-version:            0.5.1
+version:            0.5.2
 synopsis:           Schnorr signatures, ECDSA, and ECDH on the elliptic curve
                     secp256k1
 license:            MIT
@@ -9,7 +9,7 @@
 maintainer:         jared@ppad.tech
 category:           Cryptography
 build-type:         Simple
-tested-with:        GHC == { 9.8.1, 9.6.4 }
+tested-with:        GHC == { 9.10.3 }
 extra-doc-files:    CHANGELOG
 description:
   Pure BIP0340-style Schnorr signatures, deterministic RFC6979 ECDSA, and
@@ -36,9 +36,9 @@
   build-depends:
       base >= 4.9 && < 5
     , bytestring >= 0.9 && < 0.13
-    , ppad-hmac-drbg >= 0.1 && < 0.2
-    , ppad-sha256 >= 0.2 && < 0.3
-    , ppad-fixed >= 0.1.2 && < 0.2
+    , ppad-hmac-drbg >= 0.1.3 && < 0.2
+    , ppad-sha256 >= 0.2.4 && < 0.3
+    , ppad-fixed >= 0.1.3 && < 0.2
     , primitive >= 0.8 && < 0.10
 
 test-suite secp256k1-tests