diff --git a/web/apps/cast/src/pages/slideshow.tsx b/web/apps/cast/src/pages/slideshow.tsx
index 60e07b725..85cf7ad23 100644
--- a/web/apps/cast/src/pages/slideshow.tsx
+++ b/web/apps/cast/src/pages/slideshow.tsx
@@ -8,6 +8,7 @@ import {
     getCastCollection,
     getLocalFiles,
     isFileEligibleForCast,
+    readCastData,
     syncPublicFiles,
 } from "services/cast";
 import { Collection } from "types/collection";
@@ -28,13 +29,10 @@ export default function Slideshow() {
 
     const syncCastFiles = async (token: string) => {
         try {
-            console.log("syncCastFiles");
-            const castToken = window.localStorage.getItem("castToken");
-            const requestedCollectionKey =
-                window.localStorage.getItem("collectionKey");
+            const { castToken, collectionKey } = readCastData();
             const collection = await getCastCollection(
+                collectionKey,
                 castToken,
-                requestedCollectionKey,
             );
             if (
                 castCollection === undefined ||
diff --git a/web/apps/cast/src/services/cast.ts b/web/apps/cast/src/services/cast.ts
index fb19d2bfd..6e0282f79 100644
--- a/web/apps/cast/src/services/cast.ts
+++ b/web/apps/cast/src/services/cast.ts
@@ -3,6 +3,7 @@ import { isNonWebImageFileExtension } from "@/media/formats";
 import { decodeLivePhoto } from "@/media/live-photo";
 import { nameAndExtension } from "@/next/file";
 import log from "@/next/log";
+import { ensure } from "@/utils/ensure";
 import ComlinkCryptoWorker from "@ente/shared/crypto";
 import { CustomError, parseSharingErrorCodes } from "@ente/shared/error";
 import HTTPService from "@ente/shared/network/HTTPService";
@@ -26,6 +27,41 @@ const ENDPOINT = getEndpoint();
 const COLLECTION_FILES_TABLE = "collection-files";
 const COLLECTIONS_TABLE = "collections";
 
+/**
+ * Save the data received after pairing with a sender into local storage.
+ *
+ * We will read in back when we start the slideshow.
+ */
+export const storeCastData = (payload: unknown) => {
+    if (!payload || typeof payload != "object")
+        throw new Error("Unexpected cast data");
+
+    // Iterate through all the keys of the payload object and save them to
+    // localStorage. We don't validate here, we'll validate when we read these
+    // values back in `readCastData`.
+    for (const key in payload) {
+        window.localStorage.setItem(key, payload[key]);
+    }
+};
+
+interface CastData {
+    /** A key to decrypt the collection we are casting. */
+    collectionKey: string;
+    /** A credential to use for fetching media files for this cast session. */
+    castToken: string;
+}
+
+/**
+ * Read back the cast data we got after pairing.
+ *
+ * Sibling of {@link storeCastData}
+ */
+export const readCastData = (): CastData => {
+    const collectionKey = ensure(window.localStorage.getItem("collectionKey"));
+    const castToken = ensure(window.localStorage.getItem("castToken"));
+    return { collectionKey, castToken };
+};
+
 const getLastSyncKey = (collectionUID: string) => `${collectionUID}-time`;
 
 export const getLocalFiles = async (
@@ -232,8 +268,8 @@ const fetchFiles = async (
 };
 
 export const getCastCollection = async (
-    castToken: string,
     collectionKey: string,
+    castToken: string,
 ): Promise<Collection> => {
     try {
         const resp = await HTTPService.get(`${ENDPOINT}/cast/info`, null, {
@@ -278,13 +314,6 @@ export const getCastCollection = async (
     }
 };
 
-export const storeCastData = (payloadObj: Object) => {
-    // iterate through all the keys in the payload object and set them in localStorage.
-    for (const key in payloadObj) {
-        window.localStorage.setItem(key, payloadObj[key]);
-    }
-};
-
 export function sortFiles(files: EnteFile[], sortAsc = false) {
     // sort based on the time of creation time of the file,
     // for files with same creation time, sort based on the time of last modification