diff --git a/src/Form/Model/ParticipantDto.php b/src/Form/Model/ParticipantDto.php
index f4e3155..5c8da02 100644
--- a/src/Form/Model/ParticipantDto.php
+++ b/src/Form/Model/ParticipantDto.php
@@ -186,7 +186,7 @@ class ParticipantDto
         $instance->lastName = $personalData->name;
         $instance->title = $personalData->title;
         $instance->gender = $personalData->gender;
-        $instance->nationality = $personalData->nationality;
+        $instance->nationality = $personalData->nationality ?: 'D';
         $instance->email = $personalData->communication?->email;
         $instance->mobile = $personalData->communication?->mobile;
         $instance->dateOfBirth = $personalData->dateOfBirth;
diff --git a/tests/Service/ParticipantPrepopulationServiceTest.php b/tests/Service/ParticipantPrepopulationServiceTest.php
index f287c9d..ea778ce 100644
--- a/tests/Service/ParticipantPrepopulationServiceTest.php
+++ b/tests/Service/ParticipantPrepopulationServiceTest.php
@@ -107,10 +107,10 @@ class ParticipantPrepopulationServiceTest extends TestCase
         $this->assertSame('Smith', $result->lastName);
         $this->assertSame('jane@example.com', $result->email);
 
-        // Optional fields should be null
+        // Optional fields should be null (except nationality which defaults to 'D')
         $this->assertNull($result->title);
         $this->assertNull($result->gender);
-        $this->assertNull($result->nationality);
+        $this->assertSame('D', $result->nationality);
         $this->assertNull($result->mobile);
         $this->assertNull($result->height);
         $this->assertNull($result->weight);