diff --git a/lib/lib/promise-to-observable.ts b/lib/lib/promise-to-observable.ts
index ab5590b9a..c4dc70a4a 100644
--- a/lib/lib/promise-to-observable.ts
+++ b/lib/lib/promise-to-observable.ts
@@ -2,8 +2,8 @@ import { Observable } from "zen-observable-ts"
 
 export interface PromiseState<T> {
   loading: boolean
-  error: Error | null
-  data: T | null
+  error?: Error | null
+  data?: T | null
 }
 
 export function promiseToObservable<T>(