diff --git a/__tests__/hiscores.test.ts b/__tests__/hiscores.test.ts
index a08003a..62c0222 100644
--- a/__tests__/hiscores.test.ts
+++ b/__tests__/hiscores.test.ts
@@ -11,7 +11,8 @@ import {
   getPlayerTableURL,
   getSkillPageURL,
   getStatsURL,
-  BOSSES
+  BOSSES,
+  INVALID_FORMAT_ERROR
 } from '../src/index';
 
 const B0ATY_NAME = 'B0ATY';
@@ -232,6 +233,16 @@ test('Parse CSV to json', () => {
   expect(parseStats(csv)).toStrictEqual(expectedOutput);
 });
 
+test('Parse CSV with unknown activity', () => {
+  const statsWithUnknownActivity = lynxTitanStats + `
+    -1,-1`;
+  expect(() => parseStats(statsWithUnknownActivity)).toThrow(INVALID_FORMAT_ERROR);
+});
+
+test('Parse invalid CSV', () => {
+  expect(() => parseStats('invalid')).toThrow(INVALID_FORMAT_ERROR);
+});
+
 describe('Get name format', () => {
   it('gets a name with a space', async () => {
     const data = await getRSNFormat(LYNX_TITAN_SPACE_NAME);
diff --git a/src/hiscores.ts b/src/hiscores.ts
index 2b3ce0d..28501b1 100644
--- a/src/hiscores.ts
+++ b/src/hiscores.ts
@@ -27,7 +27,8 @@ import {
   rsnFromElement,
   getActivityPageURL,
   httpGet,
-  BOSSES
+  BOSSES,
+  INVALID_FORMAT_ERROR
 } from './utils';
 
 /**
@@ -74,6 +75,10 @@ export function parseStats(csv: string): Stats {
     .filter((entry) => !!entry)
     .map((stat) => stat.split(','));
 
+  if (splitCSV.length !== SKILLS.length + BH_MODES.length + CLUES.length + BOSSES.length + 3) {
+    throw Error(INVALID_FORMAT_ERROR);
+  }
+
   const skillObjects: Skill[] = splitCSV
     .filter((stat) => stat.length === 3)
     .map((stat) => {
diff --git a/src/utils/constants.ts b/src/utils/constants.ts
index 617a63a..037f437 100644
--- a/src/utils/constants.ts
+++ b/src/utils/constants.ts
@@ -245,3 +245,5 @@ export const FORMATTED_BH_NAMES: FormattedBHNames = {
 export const FORMATTED_LMS = 'Last Man Standing';
 export const FORMATTED_SOUL_WARS = 'Soul Wars Zeal';
 export const FORMATTED_LEAGUE_POINTS = 'League Points';
+
+export const INVALID_FORMAT_ERROR = 'Invalid hiscores format';