diff --git a/chapter-07/filter/src/debounceTime/random.js b/chapter-07/filter/src/debounceTime/random.js
index 7eb043c..9f41cf9 100644
--- a/chapter-07/filter/src/debounceTime/random.js
+++ b/chapter-07/filter/src/debounceTime/random.js
@@ -4,6 +4,7 @@ import 'rxjs/add/operator/filter';
 import 'rxjs/add/operator/take';
 import 'rxjs/add/operator/concat';
 import 'rxjs/add/operator/debounceTime';
+import 'rxjs/add/operator/mapTo';
 
 const source$ = Observable.interval(500).take(2).mapTo('A')
   .concat(Observable.interval(1000).take(3).mapTo('B'))