diff --git a/src/subscriber.ts b/src/subscriber.ts
index c12996862..d4ec0cc15 100644
--- a/src/subscriber.ts
+++ b/src/subscriber.ts
@@ -1000,7 +1000,7 @@ export class Subscriber extends EventEmitter {
     const shutdownStart = Date.now();
     if (
       behavior === SubscriberCloseBehaviors.WaitForProcessing &&
-      !this._inventory.isEmpty
+      !this._inventory.isEmpty()
     ) {
       const waitTimeout = timeout.subtract(FINAL_NACK_TIMEOUT);