diff --git a/include/r3.h b/include/r3.h
index 649f224..9e9721a 100644
--- a/include/r3.h
+++ b/include/r3.h
@@ -153,7 +153,7 @@ node * r3_tree_insert_pathl_(node *tree, char *path, int path_len, route * route
 
 void r3_tree_dump(const node * n, int level);
 
-int r3_tree_render_file(node * tree, char * format, char * filename);
+int r3_tree_render_file(const node * tree, const char * format, char * filename);
 
 int r3_tree_render_dot(node * tree);
 
diff --git a/src/gvc.c b/src/gvc.c
index 6f75112..cb7c7ab 100644
--- a/src/gvc.c
+++ b/src/gvc.c
@@ -72,7 +72,7 @@ int r3_tree_render_dot(node * tree)
 /**
  * Render a tree to tree graph image via graphviz (dot)
  */
-int r3_tree_render_file(node * tree, char * format, char * filename)
+int r3_tree_render_file(const node * tree, const char * format, char * filename)
 {
     Agraph_t *g;