diff --git a/src/gfile.ml b/src/gfile.ml
index 2d8aee7..9599cc3 100644
--- a/src/gfile.ml
+++ b/src/gfile.ml
@@ -45,9 +45,14 @@ let read_node id graph line =
     Printf.printf "Cannot read node in line - %s:\n%s\n%!" (Printexc.to_string e) line ;
     failwith "from_file"
 
+(* Ensure that the given node exists in the graph. If not, create it. 
+ * (Necessary because the website we use to create online graphs does not generate correct files when some nodes have been deleted.) *)
+let ensure graph id = if node_exists graph id then graph else new_node graph id
+
 (* Reads a line with an arc. *)
 let read_arc graph line =
-  try Scanf.sscanf line "e %d %d %s" (fun id1 id2 label -> new_arc graph id1 id2 label)
+  try Scanf.sscanf line "e %d %d %s"
+        (fun id1 id2 label -> new_arc (ensure (ensure graph id1) id2) id1 id2 label)
   with e ->
     Printf.printf "Cannot read arc in line - %s:\n%s\n%!" (Printexc.to_string e) line ;
     failwith "from_file"
diff --git a/src/graph.mli b/src/graph.mli
index 626ce55..416e158 100644
--- a/src/graph.mli
+++ b/src/graph.mli
@@ -17,9 +17,10 @@ val empty_graph: 'a graph
  * @raise Graph_error if the id already exists. *)
 val new_node: 'a graph -> id -> 'a graph
 
-(* add_arc gr id1 id2 lbl  : adds an arc from node id1 to node id2 with label lbl
- * If an arc already exists between id1 and id2, its label is replaced by lbl. 
- * @raise Graph_error if id1 or id2 does not exist in the graph. *)
+(* new_arc gr id1 id2 lbl  : adds an arc from node id1 to node id2 with label lbl
+ * Both nodes must already exist in the graph.
+ * If the arc already exists, its label is replaced by lbl. 
+ * @raise Graph_error if node id1 or id2 does not exist in the graph. *)
 val new_arc: 'a graph -> id -> id -> 'a -> 'a graph