diff --git a/bn_mp_radix_smap.c b/bn_mp_radix_smap.c
index 2f5de08..6e9f64a 100644
--- a/bn_mp_radix_smap.c
+++ b/bn_mp_radix_smap.c
@@ -14,7 +14,7 @@
  */
 
 /* chars used in radix conversions */
-const char *mp_s_rmap = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz+/";
+const char *const mp_s_rmap = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz+/";
 const uint8_t mp_s_rmap_reverse[] = {
    0xff, 0xff, 0xff, 0x3e, 0xff, 0xff, 0xff, 0x3f, /* ()*+,-./ */
    0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, /* 01234567 */
diff --git a/tommath_private.h b/tommath_private.h
index bc63326..133aea9 100644
--- a/tommath_private.h
+++ b/tommath_private.h
@@ -73,7 +73,7 @@ int mp_exptmod_fast(const mp_int *G, const mp_int *X, const mp_int *P, mp_int *Y
 int s_mp_exptmod(const mp_int *G, const mp_int *X, const mp_int *P, mp_int *Y, int redmode);
 void bn_reverse(unsigned char *s, int len);
 
-extern const char *mp_s_rmap;
+extern const char *const mp_s_rmap;
 extern const uint8_t mp_s_rmap_reverse[];
 extern const size_t mp_s_rmap_reverse_sz;