diff --git a/src/index.test.ts b/src/index.test.ts
index 66c6250..5816e06 100644
--- a/src/index.test.ts
+++ b/src/index.test.ts
@@ -2,7 +2,8 @@ import {
   assertStrictEquals,
   assertThrows,
 } from "https://deno.land/std@0.87.0/testing/asserts.ts";
-import { sayHello, ValidationError } from "./index.ts";
+import { sayHello } from "./index.ts";
+import { ValidationError } from "./validation.ts";
 
 Deno.test("Hello test", () => {
   const actual = sayHello();
@@ -22,3 +23,8 @@ 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 = 666;
+  assertThrows(() => sayHello(id), ValidationError, "Invalid index: 666");
+});
diff --git a/src/index.ts b/src/index.ts
index 2b1c5a6..9cfe9ce 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,10 +1,13 @@
-export class ValidationError extends Error {}
+import { ValidationError } from "./validation.ts";
+
 /**
  * Display the iconic Hello, World
  */
 export const sayHello = (id?: number) => {
-  if (id && id < 1) {
+  if (id && idInValidRange(id)) {
     throw new ValidationError(`Invalid index: ${id}`);
   }
   return (id === 5) ? "Bonjour le monde" : "Hello, World!";
 };
+
+const idInValidRange = (id: number) => id < 1 || id > 100;
diff --git a/src/validation.ts b/src/validation.ts
new file mode 100644
index 0000000..bdbb7a2
--- /dev/null
+++ b/src/validation.ts
@@ -0,0 +1,7 @@
+/**
+ * Custom error used to express input validation.
+ * The message must describe to the external user how to correct its input.
+ *
+ @example * `throw new ValidationError("Bad id")`
+ */
+export class ValidationError extends Error {}