diff --git a/src/observables/DeferObservable.ts b/src/observables/DeferObservable.ts
index 93f39c20c2..d617875293 100644
--- a/src/observables/DeferObservable.ts
+++ b/src/observables/DeferObservable.ts
@@ -6,7 +6,7 @@ import {errorObject} from '../util/errorObject';
 
 export default class DeferObservable<T> extends Observable<T> {
 
-  static create<T>(observableFactory) {
+  static create<T>(observableFactory: () => Observable<T>): Observable<T> {
     return new DeferObservable(observableFactory);
   }