diff --git a/app/Services/PurchaseOrder/ApplyNumber.php b/app/Services/PurchaseOrder/ApplyNumber.php
index 5e89c5e95bd0..4c01b9ecc30d 100644
--- a/app/Services/PurchaseOrder/ApplyNumber.php
+++ b/app/Services/PurchaseOrder/ApplyNumber.php
@@ -45,7 +45,7 @@ class ApplyNumber extends AbstractService
                 $this->trySaving();
                 break;
             case 'when_sent':
-                if ($this->invoice->status_id == PurchaseOrder::STATUS_SENT) {
+                if ($this->purchase_order->status_id == PurchaseOrder::STATUS_SENT) {
                     $this->trySaving();
                 }
                 break;