diff --git a/pages/api/__tests__/score.unit.test.js b/pages/api/__tests__/score.unit.test.js
index 91a5733..3f08538 100644
--- a/pages/api/__tests__/score.unit.test.js
+++ b/pages/api/__tests__/score.unit.test.js
@@ -16,4 +16,72 @@ describe('score endpoint', () => {
       expect(res.end).toHaveBeenCalledWith('Method not allowed');
     });
   });
+  describe('when called with a POST method', () => {
+    const emptyBody = {
+      jsonA: '{}',
+      jsonB: '{}',
+    };
+
+    const res = { end: jest.fn(), json: jest.fn() };
+    const req = {
+      method: 'POST',
+      body: emptyBody,
+    };
+    beforeEach(() => {
+      req.body = emptyBody;
+    });
+
+    it('should answer 200 ok', async () => {
+      // act
+      score(req, res);
+
+      // assert
+      expect(res.statusCode).toEqual(200);
+    });
+    it('should return a json object with the basic keys', async () => {
+      // act
+      score(req, res);
+
+      // assert
+      expect(res.json).toHaveBeenCalledWith({
+        children: [],
+        count: 0,
+        matched: 0,
+        metaData: {
+          key: '/',
+        },
+        scoreNumber: 1,
+      });
+    });
+    describe.each([
+      'jsonA',
+      'jsonB',
+    ])('when param %s is not present', (json) => {
+      it('should return a 422 explaining the missing param', async () => {
+        // arrange
+        req.body[json] = undefined;
+        // act
+        score(req, res);
+
+        // assert
+        expect(res.statusCode).toEqual(422);
+        expect(res.end).toHaveBeenCalledWith(`Missing Param: Body must contain ${json}`);
+      });
+    });
+    describe.each([
+      'jsonA',
+      'jsonB',
+    ])('when param %s is not JSON-formatted', (json) => {
+      it('should return a 422 explaining the missing param', async () => {
+        // arrange
+        req.body[json] = 'some non-json string';
+        // act
+        score(req, res);
+
+        // assert
+        expect(res.statusCode).toEqual(422);
+        expect(res.end).toHaveBeenCalledWith(`Could not parse ${json} param as a JSON object`);
+      });
+    });
+  });
 });
diff --git a/pages/api/score.js b/pages/api/score.js
index 8f385fc..2d74bbe 100644
--- a/pages/api/score.js
+++ b/pages/api/score.js
@@ -1,10 +1,42 @@
 import { calculateScore } from '~/utils/score';
 
+function isJson(str) {
+  try {
+    JSON.parse(str);
+  } catch (e) {
+    return false;
+  }
+  return true;
+}
+
+/**
+ * Endpoint that triggers the calculateScore function, checks that params are correct (jsonA and jsonB are JSON objects)
+ * @param req
+ * @param res
+ * @return {*|void|Promise<any>}
+ */
 export default (req, res) => {
   if (req.method === 'POST') {
-    res.statusCode = 200;
-    res.json(calculateScore(JSON.parse(req.body.jsonA), JSON.parse(req.body.jsonB)));
+    const validParams = ['jsonA', 'jsonB'].map((json) => {
+      res.statusCode = 422;
+      if (!req.body[json]) {
+        res.end(`Missing Param: Body must contain ${json}`);
+        return false;
+      }
+      if (!isJson(req.body[json])) {
+        res.end(`Could not parse ${json} param as a JSON object`);
+        return false;
+      }
+      return true;
+    });
+
+    if (validParams.every((isValid) => isValid === true)) {
+      res.statusCode = 200;
+      return res.json(calculateScore(JSON.parse(req.body.jsonA), JSON.parse(req.body.jsonB)));
+    }
+  } else {
+    res.statusCode = 405;
+    return res.end('Method not allowed');
   }
-  res.statusCode = 405;
-  res.end('Method not allowed');
+  return null;
 };