diff --git a/app/Observers/InvoiceObserver.php b/app/Observers/InvoiceObserver.php
index 2c86548..43f0a7c 100644
--- a/app/Observers/InvoiceObserver.php
+++ b/app/Observers/InvoiceObserver.php
@@ -23,7 +23,7 @@ public function creating(Invoice $invoice): void
     public function created(Invoice $invoice): void
     {
 
-        $invoice->internal_id = 'INV4'.str_pad($invoice->id, 4, '0', STR_PAD_LEFT);
+        $invoice->internal_id = 'TN4'.str_pad($invoice->id, 4, '0', STR_PAD_LEFT);
         $invoice->saveQuietly();
 
         $invoice->calculateTotals();