diff --git a/.github/demo.png b/.github/demo.png
new file mode 100644
index 0000000..e5b6eb9
Binary files /dev/null and b/.github/demo.png differ
diff --git a/README.md b/README.md
index aeb7ae7..dfa2c4c 100644
--- a/README.md
+++ b/README.md
@@ -6,6 +6,8 @@
 
 </div>
 
+![./demo](.github/demo.png)
+
 # Install
 
 1. `opam install . --deps-only`
diff --git a/lib/eval/eval.ml b/lib/eval/eval.ml
index c19e3d5..a6f306a 100644
--- a/lib/eval/eval.ml
+++ b/lib/eval/eval.ml
@@ -80,7 +80,9 @@ let rec eval_expr expr env =
 and eval_apply fn_expr args env =
   match fn_expr with
   | Object.Primitive (_, fn) -> fn args
-  | Object.Closure (_, names, expr, clenv) ->
+  | Object.Closure (fn_name, names, expr, clenv) ->
+    (* Check if the closure exists *)
+    Object.lookup (fn_name, env) |> ignore;
     eval_closure names expr args clenv env
   | fn_expr ->
     raise (Errors.Parse_error_exn (Apply_error (Object.string_object fn_expr)))
diff --git a/test/end_normal_test.mlisp b/test/end_normal_test.mlisp
index d57f27f..1bb24dc 100644
--- a/test/end_normal_test.mlisp
+++ b/test/end_normal_test.mlisp
@@ -76,4 +76,4 @@
 
 (letrec ((f (lambda (x) (g (+ x 1))))
          (g (lambda (x) (+ x 3))))                                                                       
-  (f 0))
+  (f 0))
\ No newline at end of file