diff --git a/Makefile b/Makefile
index 3fcc48b..8060b47 100644
--- a/Makefile
+++ b/Makefile
@@ -2,4 +2,4 @@
 run:
 	deno run --watch --unstable src/index.ts
 tests:
-	deno test
\ No newline at end of file
+	deno test --coverage --unstable
\ No newline at end of file
diff --git a/src/index.test.ts b/src/index.test.ts
index 4111cac..66c6250 100644
--- a/src/index.test.ts
+++ b/src/index.test.ts
@@ -1,5 +1,8 @@
-import { assertStrictEquals } from "https://deno.land/std@0.87.0/testing/asserts.ts";
-import { sayHello } from "./index.ts";
+import {
+  assertStrictEquals,
+  assertThrows,
+} from "https://deno.land/std@0.87.0/testing/asserts.ts";
+import { sayHello, ValidationError } from "./index.ts";
 
 Deno.test("Hello test", () => {
   const actual = sayHello();
@@ -7,10 +10,15 @@ Deno.test("Hello test", () => {
   assertStrictEquals(actual, expected);
 });
 
-Deno.test("Random Hello", () => {
+Deno.test("Specific Hello", () => {
   const id = 5;
   const actual = sayHello(id);
   const expected = "Bonjour le monde";
 
   assertStrictEquals(actual, expected);
 });
+
+Deno.test("Hello fails for non strictly positive values", () => {
+  const id = -1;
+  assertThrows(() => sayHello(id), ValidationError, "Invalid index: -1");
+});
diff --git a/src/index.ts b/src/index.ts
index b7bf741..2b1c5a6 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,8 +1,10 @@
+export class ValidationError extends Error {}
 /**
  * Display the iconic Hello, World
  */
 export const sayHello = (id?: number) => {
+  if (id && id < 1) {
+    throw new ValidationError(`Invalid index: ${id}`);
+  }
   return (id === 5) ? "Bonjour le monde" : "Hello, World!";
 };
-
-console.log(sayHello());