diff --git a/lib/hcrypto/libtommath/bn_error.c b/lib/hcrypto/libtommath/bn_error.c
index b1b7177e6..fbba7aa1f 100644
--- a/lib/hcrypto/libtommath/bn_error.c
+++ b/lib/hcrypto/libtommath/bn_error.c
@@ -17,7 +17,7 @@
 
 static const struct {
      int code;
-     char *msg;
+     const char *msg;
 } msgs[] = {
      { MP_OKAY, "Successful" },
      { MP_MEM,  "Out of heap" },
@@ -25,7 +25,7 @@ static const struct {
 };
 
 /* return a char * string for a given code */
-char *mp_error_to_string(int code)
+const char *mp_error_to_string(int code)
 {
    int x;
 
diff --git a/lib/hcrypto/libtommath/tommath.h b/lib/hcrypto/libtommath/tommath.h
index 84d3f7a59..0517b22c8 100644
--- a/lib/hcrypto/libtommath/tommath.h
+++ b/lib/hcrypto/libtommath/tommath.h
@@ -189,8 +189,8 @@ typedef int ltm_prime_callback(unsigned char *dst, int len, void *dat);
 #define DIGIT(m,k) ((m)->dp[(k)])
 #define SIGN(m)    ((m)->sign)
 
-/* error code to char* string */
-char *mp_error_to_string(int code);
+/* error code to const char* string */
+const char *mp_error_to_string(int code);
 
 /* ---> init and deinit bignum functions <--- */
 /* init a bignum */