diff --git a/README.md b/README.md
index 776f797..4d06019 100644
--- a/README.md
+++ b/README.md
@@ -4,6 +4,6 @@ Web app which returns the iconic Hello, World in various locales
 
 ## Features
 
-- [ ] Print hello world in a random locale
-- [ ] Select the locale by id
+- [x] Print hello world in a random locale
+- [x] Select the locale by id
 - [ ] Serve via internet
diff --git a/src/index.test.ts b/src/index.test.ts
index 341d2e9..c7348f7 100644
--- a/src/index.test.ts
+++ b/src/index.test.ts
@@ -1,12 +1,8 @@
-import {
-  assertStrictEquals,
-  assertThrows,
-} from "https://deno.land/std@0.87.0/testing/asserts.ts";
+import { assertStrictEquals } from "https://deno.land/std@0.87.0/testing/asserts.ts";
 import { sayHello, sayRandomHello } from "./index.ts";
-import { ValidationError } from "./validation.ts";
-import { localesSize } from "./repository/locales.ts";
+import { ID } from "./types.ts";
 
-Deno.test("Hello test", () => {
+Deno.test("Random Hello", () => {
   const result = sayRandomHello();
   const actual = typeof result;
   const expected = "string";
@@ -14,11 +10,11 @@ Deno.test("Hello test", () => {
 });
 
 Deno.test("Specific Hello", () => {
-  type TestCase = { in: number; want: string };
+  type TestCase = { in: ID; want: string };
 
   const testCases: TestCase[] = [
-    { in: 25, want: "Salut le Monde!" },
-    { in: 21, want: "Hello World!" },
+    { in: new ID(25), want: "Salut le Monde!" },
+    { in: new ID(21), want: "Hello World!" },
   ];
 
   for (const test of testCases) {
@@ -28,17 +24,3 @@ Deno.test("Specific Hello", () => {
     assertStrictEquals(actual, expected);
   }
 });
-
-Deno.test("Hello fails for non strictly positive values", () => {
-  const id = -1;
-  assertThrows(() => sayHello(id), ValidationError, "Invalid index: -1");
-});
-
-Deno.test("Hello fails for too large values", () => {
-  const id = localesSize;
-  assertThrows(
-    () => sayHello(id),
-    ValidationError,
-    `Invalid index: ${localesSize}`,
-  );
-});
diff --git a/src/index.ts b/src/index.ts
index c4f9613..61fad20 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,21 +1,19 @@
-import { idIsValid, ValidationError } from "./validation.ts";
 import { getLocaleById, localesSize } from "./repository/locales.ts";
+import { ID } from "./types.ts";
 
 /**
  * Display the iconic Hello, World in a random locale
  */
 export const sayRandomHello = () => {
   const id = Math.ceil(Math.random() * localesSize);
-  return sayHello(id);
+  const val = new ID(id);
+  return sayHello(val);
 };
 
 /**
  * Display the iconic Hello, World in a locale identified by id
  * @param id Unique identifier
  */
-export const sayHello = (id: number): string => {
-  if (!idIsValid(id)) {
-    throw new ValidationError(`Invalid index: ${id}`);
-  }
-  return getLocaleById(id);
+export const sayHello = (id: ID): string => {
+  return getLocaleById(id.id);
 };
diff --git a/src/types.test.ts b/src/types.test.ts
new file mode 100644
index 0000000..395ff89
--- /dev/null
+++ b/src/types.test.ts
@@ -0,0 +1,16 @@
+import { ID } from "./types.ts";
+import { ValidationError } from "./validation.ts";
+import { assertThrows } from "https://deno.land/std@0.87.0/testing/asserts.ts";
+import { localesSize } from "./repository/locales.ts";
+
+Deno.test("ID validation fails for non strictly positive values", () => {
+  assertThrows(() => new ID(-1), ValidationError, "Invalid index: -1");
+});
+
+Deno.test("Hello fails for too large values", () => {
+  assertThrows(
+    () => new ID(localesSize),
+    ValidationError,
+    `Invalid index: ${localesSize}`,
+  );
+});
diff --git a/src/types.ts b/src/types.ts
new file mode 100644
index 0000000..e548fe9
--- /dev/null
+++ b/src/types.ts
@@ -0,0 +1,16 @@
+import { ValidationError } from "./validation.ts";
+import { localesSize } from "./repository/locales.ts";
+
+export class ID {
+  constructor(public id: number) {
+    this.#isValid(id);
+  }
+
+  #idIsValid = (id: number) => id > 0 && id < localesSize;
+
+  #isValid = (id: number) => {
+    if (!this.#idIsValid(id)) {
+      throw new ValidationError(`Invalid index: ${id}`);
+    }
+  };
+}
diff --git a/src/validation.ts b/src/validation.ts
index f3a52c9..89bdf8b 100644
--- a/src/validation.ts
+++ b/src/validation.ts
@@ -4,8 +4,5 @@
  *
  @example * `throw new ValidationError("Bad id")`
  */
-import { localesSize } from "./repository/locales.ts";
 
 export class ValidationError extends Error {}
-
-export const idIsValid = (id: number) => id > 0 && id < localesSize;