diff --git a/CHANGES.md b/CHANGES.md
index e7c9fa86..d31de8c6 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,4 +1,6 @@
 
+  - #31: fixed documentation of [map_vertex]: the supplied function
+    must be injective
   - #110: ensure that map_vertex applies the function only once per vertex
 
 # 2.0.0 (October 2, 2020)
diff --git a/src/sig.mli b/src/sig.mli
index abe28f94..b77e8cbd 100644
--- a/src/sig.mli
+++ b/src/sig.mli
@@ -189,7 +189,13 @@ module type G = sig
   (** Fold on all edges of a graph. *)
 
   val map_vertex : (vertex -> vertex) -> t -> t
-  (** Map on all vertices of a graph. *)
+  (** Map on all vertices of a graph.
+
+      The current implementation requires the supplied function to be
+      injective. Said otherwise, [map_vertex] cannot be used to contract
+      a graph by mapping several vertices to the same vertex.
+      To contract a graph, use instead [create], [add_vertex],
+      and [add_edge]. *)
 
   (** {2 Vertex iterators}
 
diff --git a/tests/test_31.ml b/tests/test_31.ml
new file mode 100644
index 00000000..2c593681
--- /dev/null
+++ b/tests/test_31.ml
@@ -0,0 +1,32 @@
+
+(* Issue #31
+
+   This would fail in the current state of OCamlGraph,
+   as map_vertex requires a supplied function that is injective.
+*)
+
+module String = struct include String let hash = Hashtbl.hash end
+module G = Graph.Persistent.Digraph.ConcreteBidirectional(String)
+
+(* Make a persistent graph where:
+     A -> B
+     B -> C
+     C -> B *)
+let g = List.fold_left (fun g (x, y) ->
+            G.add_edge g x y) G.empty [("A", "B"); ("B", "C"); ("C", "B")]
+
+(* Contract the SCC *)
+module C = Graph.Components.Make(G)
+
+let contract g =
+  let names = Array.map (String.concat ",") (C.scc_array g) in
+  let (_, numbering) = C.scc g in
+  Array.fold_left (fun g x -> G.remove_edge g x x)
+    (G.map_vertex (fun x -> names.(numbering x)) g)
+    names
+
+let g' = contract g
+(* this is now "A" -> "C,B" *)
+
+let () =
+  assert (G.in_degree g' "C,B" = 1)