update replaceNumberToString, so it will change ONLY numbers to string

CHANGELOG.md

@@ -22,6 +22,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - add ssrAppId to avoid second meta render on csr - @gibkigonzo (#4203)
 - take control over default broswer behavior and use saved category page size to load prev products - @gibkigonzo (#4201)
 - update getCurrentCartHash after add/remove coupon - @gibkigonzo (#4220)
+- update replaceNumberToString, so it will change ONLY numbers to string - @gibkigonzo (#4217)
 
 ## [1.11.2] - 2020.03.10
 

core/modules/cart/helpers/productChecksum.ts

@@ -6,10 +6,11 @@ import cloneDeep from 'lodash-es/cloneDeep';
 
 const replaceNumberToString = obj => {
   Object.keys(obj).forEach(key => {
-    if (typeof obj[key] === 'object') {
+    if (obj[key] !== null && typeof obj[key] === 'object') {
       return replaceNumberToString(obj[key]);
+    } else if (typeof obj[key] === 'number') {
+      obj[key] = String(obj[key]);
     }
-    obj[key] = String(obj[key]);
   });
   return obj;
 }