diff --git a/src/services/NotificationService.ts b/src/services/NotificationService.ts
index 65a2f0a..51e56af 100644
--- a/src/services/NotificationService.ts
+++ b/src/services/NotificationService.ts
@@ -19,6 +19,14 @@ export class NotificationService {
 
   constructor(private repository: GetterRepository) {}
 
+  /**
+   * An object of device ID arrays to deliver notifications to.
+   * The key should be a combination of the shuttle ID and
+   * stop ID, which can be generated using `TupleKey`.
+   * @private
+   */
+  private deviceIdsToDeliverTo: { [key: string]: string[] } = {}
+
   public reloadAPNsTokenIfTimePassed() {
     if (this.lastReloadedTimeForAPNsIsTooRecent()) {
       return;