diff --git a/src/mqtt.service.ts b/src/mqtt.service.ts
index 59d3be5..b096946 100644
--- a/src/mqtt.service.ts
+++ b/src/mqtt.service.ts
@@ -144,14 +144,16 @@ export class MqttService {
           () => {
             const subscription: Subscription = new Subscription();
             this.client.subscribe(filterString, (err, granted: ISubscriptionGrant[]) => {
-              granted.forEach((granted_: ISubscriptionGrant) => {
-                if (granted_.qos === 128) {
-                  delete this.observables[granted_.topic];
-                  this.client.unsubscribe(granted_.topic);
-                  rejected.error(`subscription for '${granted_.topic}' rejected!`);
-                }
-                this._onSuback.emit({ filter: filterString, granted: granted_.qos !== 128 });
-              });
+              if(granted) { // granted can be undefined when an error occurs when the client is disconnecting
+                  granted.forEach((granted_: ISubscriptionGrant) => {
+                      if (granted_.qos === 128) {
+                          delete this.observables[granted_.topic];
+                          this.client.unsubscribe(granted_.topic);
+                          rejected.error(`subscription for '${granted_.topic}' rejected!`);
+                      }
+                      this._onSuback.emit({filter: filterString, granted: granted_.qos !== 128});
+                  });
+              }
             });
             subscription.add(() => {
               delete this.observables[filterString];