diff --git a/src/int128_struct_impl.h b/src/int128_struct_impl.h
index 5e64ac41987a6..b5f8fb7b650c9 100644
--- a/src/int128_struct_impl.h
+++ b/src/int128_struct_impl.h
@@ -19,7 +19,7 @@ static SECP256K1_INLINE uint64_t secp256k1_umul128(uint64_t a, uint64_t b, uint6
 
 static SECP256K1_INLINE int64_t secp256k1_mul128(int64_t a, int64_t b, int64_t* hi) {
     *hi = __mulh(a, b);
-    return a * b;
+    return (uint64_t)a * (uint64_t)b;
 }
 #    else
 /* On x84_64 MSVC, use native _(u)mul128 for 64x64->128 multiplications. */