diff --git a/bn_error.c b/bn_error.c
index ad32d1c..03a0e7a 100644
--- a/bn_error.c
+++ b/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;