diff --git a/tommath.h b/tommath.h
index a5fceec..1baff1f 100644
--- a/tommath.h
+++ b/tommath.h
@@ -190,7 +190,7 @@ typedef int ltm_prime_callback(unsigned char *dst, int len, void *dat);
 #define SIGN(m)    ((m)->sign)
 
 /* error code to char* string */
-char *mp_error_to_string(int code);
+const char *mp_error_to_string(int code);
 
 /* ---> init and deinit bignum functions <--- */
 /* init a bignum */