diff --git a/src/Iter.ml b/src/Iter.ml
index cfec399..7060895 100644
--- a/src/Iter.ml
+++ b/src/Iter.ml
@@ -114,7 +114,7 @@ let map_by_2 f seq k =
   let f y =
     match !r with
     | None -> r := Some y
-    | Some x -> k (f x y)
+    | Some x -> r := None; k (f x y)
   in
   seq f;
   match !r with
diff --git a/tests/unit/t_iter.ml b/tests/unit/t_iter.ml
index beaf8d6..a710aba 100644
--- a/tests/unit/t_iter.ml
+++ b/tests/unit/t_iter.ml
@@ -347,3 +347,21 @@ let () =
 let () =
   let errcode = QCheck_base_runner.run_tests ~colors:false !qchecks in
   if errcode <> 0 then exit errcode
+
+(* map_by_2 tests *)
+let test = OUnit.assert_equal ~printer:Q.Print.(list int)
+let () = test [] (map_by_2 (fun a _ -> a) (of_list []) |> to_list)
+(* Test empty iterator *)
+let () = test [1] (map_by_2 (fun _ b -> b) (of_list [1]) |> to_list)
+let () = test [3] (map_by_2 (fun _ b -> b) (1--3) |> drop 1 |> to_list)
+let () = test [9] (map_by_2 (fun _ b -> b) (1--9) |> drop 4 |> to_list)
+(* Odd number of elements should leave the last element in the iterator.
+   For an increasing integer range [1,2k] (fun _ b -> b) returns only
+   even numbers so this is sufficient to test that this element is left
+   in the iterator. *)
+let () = test [1] (map_by_2 (fun a _ -> a) (1--2) |> to_list)
+let () = test [2] (map_by_2 (fun _ b -> b) (1--2) |> to_list)
+(* Test two elements *)
+let () = test [1;3;5;7;9] (map_by_2 (fun a _ -> a) (1--10) |> to_list)
+let () = test [2;4;6;8;10] (map_by_2 (fun _ b -> b) (1--10) |> to_list)
+(* Test more than two elements *)