diff --git a/src/operators/groupBy.ts b/src/operators/groupBy.ts
index bdc287dd45..0cfbea43b6 100644
--- a/src/operators/groupBy.ts
+++ b/src/operators/groupBy.ts
@@ -116,7 +116,7 @@ class GroupBySubscriber<T, R> extends Subscriber<T> {
   }
 
   removeGroup(key: string) {
-    this.groups[key] = null;
+    this.groups.delete(key);
   }
 }
 
diff --git a/src/util/FastMap.ts b/src/util/FastMap.ts
index 448944fd2b..8fbeb31cdd 100644
--- a/src/util/FastMap.ts
+++ b/src/util/FastMap.ts
@@ -19,7 +19,7 @@ export default class FastMap {
   forEach(cb, thisArg) {
     const values = this._values;
     for (let key in values) {
-      if (values.hasOwnProperty(key)) {
+      if (values.hasOwnProperty(key) && values[key] !== null) {
         cb.call(thisArg, values[key], key);
       }
     }