diff --git a/include/r3.h b/include/r3.h
index 66f2207..8334a49 100644
--- a/include/r3.h
+++ b/include/r3.h
@@ -122,13 +122,13 @@ edge * r3_node_find_edge(const node * n, const char * pat, int pat_len);
 void r3_node_append_edge(node *n, edge *child);
 
 
-edge * r3_node_find_common_prefix(node *n, const char *path, int path_len, int *prefix_len, const char **errstr);
+edge * r3_node_find_common_prefix(node *n, const char *path, int path_len, int *prefix_len, char **errstr);
 
 node * r3_tree_insert_pathl(node *tree, const char *path, int path_len, void * data);
 
 route * r3_tree_insert_routel(node *tree, int method, const char *path, int path_len, void *data);
 
-route * r3_tree_insert_routel_ex(node *tree, int method, const char *path, int path_len, void *data, const char **errstr);
+route * r3_tree_insert_routel_ex(node *tree, int method, const char *path, int path_len, void *data, char **errstr);
 
 #define r3_tree_insert_routel(n, method, path, path_len, data) r3_tree_insert_routel_ex(n, method, path, path_len, data, NULL)
 
@@ -140,7 +140,7 @@ route * r3_tree_insert_routel_ex(node *tree, int method, const char *path, int p
 /**
  * The private API to insert a path
  */
-node * r3_tree_insert_pathl_ex(node *tree, const char *path, int path_len, route * route, void * data, const char ** errstr);
+node * r3_tree_insert_pathl_ex(node *tree, const char *path, int path_len, route * route, void * data, char ** errstr);
 
 void r3_tree_dump(const node * n, int level);
 
diff --git a/src/node.c b/src/node.c
index 259ce29..97c40a2 100644
--- a/src/node.c
+++ b/src/node.c
@@ -473,7 +473,7 @@ route * r3_route_createl(const char * path, int path_len) {
  *
  * method (int): METHOD_GET, METHOD_POST, METHOD_PUT, METHOD_DELETE ...
  */
-route * r3_tree_insert_routel_ex(node *tree, int method, const char *path, int path_len, void *data, const char **errstr) {
+route * r3_tree_insert_routel_ex(node *tree, int method, const char *path, int path_len, void *data, char **errstr) {
     route *r = r3_route_createl(path, path_len);
     CHECK_PTR(r);
     r->request_method = method; // ALLOW GET OR POST METHOD
@@ -506,7 +506,7 @@ node * r3_tree_insert_pathl(node *tree, const char *path, int path_len, void * d
  * 4.  "aaa{slug:xxx}/hate" vs "aab{slug:yyy}/bar"      => common prefix = "aa"
  * 5.  "/foo/{slug}/hate" vs "/fo{slug}/bar"            => common prefix = "/fo"
  */
-edge * r3_node_find_common_prefix(node *n, const char *path, int path_len, int *prefix_len, const char **errstr) {
+edge * r3_node_find_common_prefix(node *n, const char *path, int path_len, int *prefix_len, char **errstr) {
     int i = 0;
     int prefix = 0;
     *prefix_len = 0;
@@ -567,7 +567,7 @@ edge * r3_node_find_common_prefix(node *n, const char *path, int path_len, int *
 /**
  * Return the last inserted node.
  */
-node * r3_tree_insert_pathl_ex(node *tree, const char *path, int path_len, route * route, void * data, const char **errstr)
+node * r3_tree_insert_pathl_ex(node *tree, const char *path, int path_len, route * route, void * data, char **errstr)
 {
     node * n = tree;
 
@@ -578,7 +578,7 @@ node * r3_tree_insert_pathl_ex(node *tree, const char *path, int path_len, route
 
     /* length of common prefix */
     int prefix_len = 0;
-    const char *err = NULL;
+    char *err = NULL;
     e = r3_node_find_common_prefix(tree, path, path_len, &prefix_len, &err);
     if (err) {
         // copy the error message pointer
diff --git a/src/slug.c b/src/slug.c
index bb07cbb..7990fc9 100644
--- a/src/slug.c
+++ b/src/slug.c
@@ -76,7 +76,7 @@ Return 1 => Slug found
 Return -1 => Slug parsing error
 */
 
-int r3_slug_parse(r3_slug_t *s, const char *needle, int needle_len, const char *offset, const char **errstr) {
+int r3_slug_parse(r3_slug_t *s, const char *needle, int needle_len, const char *offset, char **errstr) {
     s->path = (char*) needle;
     s->path_len = needle_len;
 
@@ -152,7 +152,7 @@ int r3_slug_parse(r3_slug_t *s, const char *needle, int needle_len, const char *
 /**
  * provide a quick way to count slugs, simply search for '{'
  */
-int slug_count(const char * needle, int len, const char **errstr) {
+int slug_count(const char * needle, int len, char **errstr) {
     int cnt = 0;
     int state = 0;
     char * p = (char*) needle;
diff --git a/src/slug.h b/src/slug.h
index e7b7618..7511125 100644
--- a/src/slug.h
+++ b/src/slug.h
@@ -43,13 +43,13 @@ r3_slug_t * r3_slug_new(const char * path, int path_len);
 
 int r3_slug_check(r3_slug_t *s);
 
-int r3_slug_parse(r3_slug_t *s, const char *needle, int needle_len, const char *offset, const char **errstr);
+int r3_slug_parse(r3_slug_t *s, const char *needle, int needle_len, const char *offset, char **errstr);
 
 char * r3_slug_to_str(const r3_slug_t *s);
 
 void r3_slug_free(r3_slug_t * s);
 
-int slug_count(const char * needle, int len, const char **errstr);
+int slug_count(const char * needle, int len, char **errstr);
 
 static inline int r3_path_contains_slug_char(const char * str) {
     return strchr(str, '{') != NULL ? 1 : 0;