diff --git a/package.json b/package.json
index 1d66644..4d18635 100644
--- a/package.json
+++ b/package.json
@@ -6,6 +6,7 @@
   "types": "dist/index.d.ts",
   "scripts": {
     "build": "tsc",
+    "install": "npm run build",
     "test": "mocha -r ts-node/register src/test/*.spec.ts"
   },
   "repository": {
diff --git a/src/functions/find-items.ts b/src/functions/find-items.ts
index 1217df7..1bf628a 100644
--- a/src/functions/find-items.ts
+++ b/src/functions/find-items.ts
@@ -18,10 +18,14 @@ export function findItems(response: any, temp: any) {
         throw '[Normalize] [JSON:API Syntax Error] A data relationship item does not contain id field!';
     }
     const result = [];
-    // Adding all found results
-    result.push(
-        response.included.find((item: any) => (String(item.id) === String(temp.id)) &&
-            (String(item.type) === String(temp.type))),
-    );
+
+    // If response has a 'included' property then look for results
+    if (response.hasOwnProperty('included')) {
+        // Adding all found results
+        result.push(
+            response.included.find((item: any) => (String(item.id) === String(temp.id)) &&
+                (String(item.type) === String(temp.type))),
+        );
+    }
     return result;
 }
diff --git a/src/index.ts b/src/index.ts
index 0e64323..922460d 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -47,6 +47,12 @@ export default function normalize(response: any) {
         // Using a for loop to go through all relationships of every item in data
         for (const key of Object.keys(item.relationships)) {
             const relation = item.relationships[key];
+
+            // Skip if relationship is not present
+            if (!relation.data) {
+                continue;
+            }
+
             // Checking if there is only one relationship stored in an object
             if (relation.data.constructor === Array) {
                 // Checking if a data relationship item has type field
diff --git a/src/test/normalize.spec.ts b/src/test/normalize.spec.ts
new file mode 100644
index 0000000..c004c8c
--- /dev/null
+++ b/src/test/normalize.spec.ts
@@ -0,0 +1,79 @@
+/*
+* Validation test file
+* Description: This file contains tests which check if any modifications
+* make the new code incompatible with the old one.
+*/
+
+// Importing the normalize function
+import normalize from '../index';
+// Importing mocha and chai for the tests
+import { expect } from 'chai';
+import 'mocha';
+
+// Creating the test set
+describe('normalize test', () => {
+    it('should normalize when included relationship is null', () => {
+        const test = {
+           "data":[
+              {
+                 "id":"3",
+                 "type":"conversation",
+                 "attributes":{
+                    "subject":"Test subject"
+                 },
+                 "relationships":{
+                    "last_message":{
+                       "data":{ type: "people", id: "9" }
+                    }
+                 }
+              },
+           ],
+           "included": [],
+        };
+        expect(() => normalize(test)).to.not.throw();
+        const normalized = normalize(test);
+        expect(normalized).to.have.property('conversation');
+    });
+    it('should normalize when property "included" is undefined', () => {
+        const test = {
+           "data":[
+              {
+                 "id":"3",
+                 "type":"conversation",
+                 "attributes":{
+                    "subject":"Test subject"
+                 },
+                 "relationships":{
+                    "last_message":{
+                       "data":{ type: "people", id: "9" }
+                    }
+                 }
+              },
+           ],
+        };
+        expect(() => normalize(test)).to.not.throw();
+        const normalized = normalize(test);
+        expect(normalized).to.have.property('conversation');
+    });
+    it('should normalize when property "included" is undefined and relationship is null', () => {
+        const test = {
+           "data":[
+              {
+                 "id":"3",
+                 "type":"conversation",
+                 "attributes":{
+                    "subject":"Test subject"
+                 },
+                 "relationships":{
+                    "last_message":{
+                       "data": null
+                    }
+                 }
+              },
+           ],
+        };
+        expect(() => normalize(test)).to.not.throw();
+        const normalized = normalize(test);
+        expect(normalized).to.have.property('conversation');
+    });
+});
diff --git a/src/test/validate.spec.ts b/src/test/validate.spec.ts
index 384674a..71bec56 100644
--- a/src/test/validate.spec.ts
+++ b/src/test/validate.spec.ts
@@ -51,4 +51,4 @@ describe('Validation test', () => {
             validate(test);
         }).to.throw('[Normalize] [JSON:API Syntax Error] Not all data items are the same type!');
     });
-});
\ No newline at end of file
+});