diff --git a/include/r3_gvc.h b/include/r3_gvc.h
index 4d6599d..1fe1f7d 100644
--- a/include/r3_gvc.h
+++ b/include/r3_gvc.h
@@ -11,7 +11,7 @@
 
 void r3_tree_build_ag_nodes(Agraph_t * g, Agnode_t * ag_parent_node, const node * n, int node_cnt);
 
-int r3_tree_render_dot(node * tree);
+int r3_tree_render_dot(const node * tree);
 
 int r3_tree_render_file(const node * tree, const char * format, const char * filename);
 
diff --git a/src/gvc.c b/src/gvc.c
index e9672a6..dc0aacd 100644
--- a/src/gvc.c
+++ b/src/gvc.c
@@ -45,7 +45,7 @@ void r3_tree_build_ag_nodes(Agraph_t * g, Agnode_t * ag_parent_node, const node
 /**
  * Render a tree to tree graph image via graphviz (dot)
  */
-int r3_tree_render_dot(node * tree)
+int r3_tree_render_dot(const node * tree)
 {
     Agraph_t *g;
     /* set up a graphviz context - but only once even for multiple graphs */