diff --git a/src/Service/Pdf/InvoiceRenderer.php b/src/Service/Pdf/InvoiceRenderer.php
index e9614ea..5051001 100644
--- a/src/Service/Pdf/InvoiceRenderer.php
+++ b/src/Service/Pdf/InvoiceRenderer.php
@@ -37,7 +37,9 @@ class InvoiceRenderer extends AbstractPdfRenderer
 
         // Invoice number
         $pdf->SetFont('Arial', '', 12);
-        $number = $assignment->getEffectivePeriod()->start->format('d/m/Y');
+        $date = $assignment->getEffectivePeriod()->start->format('d/m/Y');
+        $suffix = $assignment->getId();
+        $number = sprintf('%s-%05d', $date, $suffix);
         $pdf->Text(85, 116, $number);
 
         $pdf->SetFont('Arial', '', 10);
diff --git a/tests/Service/Pdf/InvoiceRendererTest.php b/tests/Service/Pdf/InvoiceRendererTest.php
index 66269f1..0f79c6d 100644
--- a/tests/Service/Pdf/InvoiceRendererTest.php
+++ b/tests/Service/Pdf/InvoiceRendererTest.php
@@ -16,6 +16,7 @@ use App\Entity\User;
 use App\Service\Pdf\InvoiceApprover;
 use App\Service\Pdf\InvoiceRenderer;
 use App\Service\Pdf\Pdf;
+use Carbon\CarbonPeriod;
 use Symfony\Bundle\FrameworkBundle\Test\WebTestCase;
 use Symfony\Contracts\Translation\TranslatorInterface;
 
@@ -81,12 +82,12 @@ class InvoiceRendererTest extends WebTestCase
             ->setEmail('carmen.baer@ep-reisen.de')
         ;
 
-        $assignment = new Assignment();
-        $assignment
-            ->setDestination($destination)
-            ->setJobProfile($jobProfile)
-            ->setContact($contact)
-        ;
+        $assignment = $this->createMock(Assignment::class);
+        $assignment->method('getEffectivePeriod')->willReturn(CarbonPeriod::create('2025-02-01', '2025-02-08'));
+        $assignment->method('getId')->willReturn(1234);
+        $assignment->method('getDestination')->willReturn($destination);
+        $assignment->method('getJobProfile')->willReturn($jobProfile);
+        $assignment->method('getContact')->willReturn($contact);
 
         $application = new Application($assignment, $teamer);
         $disposition = new Disposition($application);