diff --git a/src/utils/helpers.ts b/src/utils/helpers.ts
index 6a19a4c..902a341 100644
--- a/src/utils/helpers.ts
+++ b/src/utils/helpers.ts
@@ -2,7 +2,7 @@ export function deepFreeze<T>(obj: T): void {
   const propNames = Object.getOwnPropertyNames(obj);
   for (const name of propNames) {
     const value = obj[name as keyof T];
-    if (typeof value === 'object') {
+    if (typeof value === 'object' && value !== null) {
       deepFreeze(value);
     }
   }
diff --git a/tests/helpers.spec.ts b/tests/helpers.spec.ts
new file mode 100644
index 0000000..a409ffb
--- /dev/null
+++ b/tests/helpers.spec.ts
@@ -0,0 +1,43 @@
+import 'jest-extended';
+import { deepFreeze } from '../src/utils/helpers';
+
+describe('helpers', () => {
+  describe('#deepFreeze', () => {
+    it('should return frozen object', () => {
+      const data = {
+        name: 'I am parent',
+        child: {
+          name: 'I am child',
+        },
+      };
+
+      deepFreeze(data);
+      expect(data).toBeFrozen();
+    });
+
+    it('should freeze the input object that include null valued property without error', () => {
+      const data = {
+        name: null,
+        child: {
+          name: 'i am child',
+        },
+      };
+
+      deepFreeze(data);
+      expect(data).toBeFrozen();
+    });
+
+    it('should freeze the input object include nested property', () => {
+      const data = {
+        name: 'I am parent',
+        child: {
+          name: 'I am child',
+        },
+      };
+
+      deepFreeze(data);
+      const child = data.child;
+      expect(child).toBeFrozen();
+    });
+  });
+});