diff --git a/libguile/numbers.c b/libguile/numbers.c
index 5ad49104c..0ac67876f 100644
--- a/libguile/numbers.c
+++ b/libguile/numbers.c
@@ -6881,14 +6881,14 @@ scm_to_mpz (SCM val, mpz_t rop)
 #if SCM_LONG_BIT >= SCM_I_FIXNUM_BIT
       // Cast to long and directly pass to GMP.
       mpz_set_si (rop, (long)inum);
-#elif (2 * SCM_LONG_BIT) > SCM_I_FIXNUM_BIT
+#elif (2 * SCM_INTPTR_T_BIT) > SCM_I_FIXNUM_BIT
       scm_t_inum inum_abs = inum;
       if (inum < 0)
         inum_abs *= -1;
-      long high = inum_abs >> (SCM_LONG_BIT - 1);
-      long low = (long)(inum_abs & ((((scm_t_inum)1) << (SCM_LONG_BIT - 1)) - 1));
+      intptr_t high = inum_abs >> (SCM_INTPTR_T_BIT - 1);
+      intptr_t low = (intptr_t)(inum_abs & ((((scm_t_inum)1) << (SCM_INTPTR_T_BIT - 1)) - 1));
       mpz_set_si (rop, high);
-      mpz_mul_2exp (rop, rop, SCM_LONG_BIT - 1);
+      mpz_mul_2exp (rop, rop, SCM_INTPTR_T_BIT - 1);
       mpz_add_ui (rop, rop, low);
       if (inum < 0)
         mpz_neg (rop, rop);