diff --git a/Project-Euler/Problem001.js b/Project-Euler/Problem001.js
index 65a35e32a..c47c10bd1 100644
--- a/Project-Euler/Problem001.js
+++ b/Project-Euler/Problem001.js
@@ -5,9 +5,11 @@ Find the sum of all the multiples of 3 or 5 below the provided parameter value n
 */
 
 const multiplesThreeAndFive = (num) => {
+  if (num < 1) throw new Error('No natural numbers exist below 1')
+
   let total = 0
   // total for calculating the sum
-  for (let i = 0; i < num; i++) {
+  for (let i = 1; i < num; i++) {
     if (i % 3 === 0 || i % 5 === 0) {
       total += i
     }
diff --git a/Project-Euler/test/Problem001.test.js b/Project-Euler/test/Problem001.test.js
new file mode 100644
index 000000000..e6b5549a5
--- /dev/null
+++ b/Project-Euler/test/Problem001.test.js
@@ -0,0 +1,17 @@
+import { multiplesThreeAndFive } from '../Problem001.js'
+
+describe('Sum of multiples of 3 or 5', () => {
+  it('should throw error when number is negative number', () => {
+    expect(() => multiplesThreeAndFive(-24)).toThrowError('No natural numbers exist below 1')
+  })
+  it('should throw error when number is 0', () => {
+    expect(() => multiplesThreeAndFive(0)).toThrowError('No natural numbers exist below 1')
+  })
+  test('if the number is greater than 0', () => {
+    expect(multiplesThreeAndFive(10)).toBe(23)
+  })
+  // Project Euler Condition Check
+  test('if the number is 1000', () => {
+    expect(multiplesThreeAndFive(1000)).toBe(233168)
+  })
+})