diff --git a/src/node.c b/src/node.c
index 956ae82..39f9cf8 100644
--- a/src/node.c
+++ b/src/node.c
@@ -516,8 +516,8 @@ edge * r3_node_find_common_prefix(node *n, const char *path, int path_len, int *
     if (prefix > 0) {
         r3_slug_t *slug;
         int ret = 0;
-        char *offset = (char*) path;
-        char *p = ((char*)path) + prefix;
+        const char *offset = (char*) path;
+        const char *p = ((char*)path) + prefix;
 
         slug = r3_slug_new(path, path_len);
 
@@ -532,7 +532,7 @@ edge * r3_node_find_common_prefix(node *n, const char *path, int path_len, int *
                 } else if ( p < slug->begin ) {
                     break;
                 } else if ( p >= slug->end && p < (path + path_len) ) {
-                    offset = (char*) slug->end + 1;
+                    offset = slug->end + 1;
                     prefix = p - path;
                     continue;
                 } else {
diff --git a/src/slug.c b/src/slug.c
index cd62b16..804c89e 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, char *offset, 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;
 
diff --git a/src/slug.h b/src/slug.h
index 14db6c2..7511125 100644
--- a/src/slug.h
+++ b/src/slug.h
@@ -43,7 +43,7 @@ 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, char *offset, 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);