diff --git a/packages/klevu-core/src/usersession.ts b/packages/klevu-core/src/usersession.ts
index 737088c8..d3e3c566 100644
--- a/packages/klevu-core/src/usersession.ts
+++ b/packages/klevu-core/src/usersession.ts
@@ -1,4 +1,4 @@
-import { KlevuConfig } from "."
+import { KlevuConfig } from "./index.js"
 import { post } from "./connection/fetch.js"
 import { Klaviyo } from "./connectors/klaviyo.js"
 import { KlevuStorage } from "./utils/index.js"
diff --git a/packages/klevu-core/src/utils/storage.ts b/packages/klevu-core/src/utils/storage.ts
index 41c2f98f..c3973a35 100644
--- a/packages/klevu-core/src/utils/storage.ts
+++ b/packages/klevu-core/src/utils/storage.ts
@@ -1,4 +1,4 @@
-import { KlevuConfig } from ".."
+import { KlevuConfig } from "../index.js"
 
 export enum StorageType {
   SESSION = "session",
@@ -6,11 +6,19 @@ export enum StorageType {
 }
 
 const isSessionStorage = (storageType: StorageType) => {
-  return storageType === StorageType.SESSION && window.sessionStorage
+  return (
+    storageType === StorageType.SESSION &&
+    typeof window !== "undefined" &&
+    window.sessionStorage
+  )
 }
 
 const isLocalStorage = (storageType: StorageType) => {
-  return storageType === StorageType.LOCAL && window.localStorage
+  return (
+    storageType === StorageType.LOCAL &&
+    typeof window !== "undefined" &&
+    window.localStorage
+  )
 }
 
 export class KlevuStorage {