diff --git a/src/BusProNet/DataProcessor/PersonalDataSynchronizer.php b/src/BusProNet/DataProcessor/PersonalDataSynchronizer.php
index a98c6da..633cc1f 100644
--- a/src/BusProNet/DataProcessor/PersonalDataSynchronizer.php
+++ b/src/BusProNet/DataProcessor/PersonalDataSynchronizer.php
@@ -7,6 +7,7 @@ namespace App\BusProNet\DataProcessor;
 use App\BusProNet\Model\Address;
 use App\BusProNet\Model\Booking;
 use App\BusProNet\Model\Communication;
+use App\BusProNet\Model\PersonalData;
 use App\Form\Model\ParticipantDto;
 
 /**
@@ -50,6 +51,8 @@ class PersonalDataSynchronizer
             $bookingData->participants[$participant->index]->dateOfBirth = $participant->dateOfBirth;
             $bookingData->participants[$participant->index]->gender = $participant->gender;
             $bookingData->participants[$participant->index]->nationality = $participant->nationality;
+
+            $this->fillMissingParticipantFields($bookingData->participants[$participant->index]);
             $bookingData->participants[$participant->index]->height = $participant->height;
             $bookingData->participants[$participant->index]->weight = $participant->weight;
             $bookingData->participants[$participant->index]->shoeSize = $participant->shoeSize;
@@ -117,4 +120,19 @@ class PersonalDataSynchronizer
             $applicant->communication->mobile = '12345';
         }
     }
+
+    /**
+     * Fills missing mandatory fields on a participant.
+     *
+     * Ensures mandatory fields have sensible defaults before submission to the
+     * BPN API. Currently handles nationality which defaults to 'D' (German).
+     *
+     * @param \App\BusProNet\Model\PersonalData $participant The participant to fill defaults for
+     */
+    private function fillMissingParticipantFields(PersonalData $participant): void
+    {
+        if (null === $participant->nationality || '' === $participant->nationality) {
+            $participant->nationality = 'D';
+        }
+    }
 }
diff --git a/src/Form/BookingEditParticipantType.php b/src/Form/BookingEditParticipantType.php
index 39fb931..ff20cb5 100644
--- a/src/Form/BookingEditParticipantType.php
+++ b/src/Form/BookingEditParticipantType.php
@@ -76,7 +76,9 @@ class BookingEditParticipantType extends AbstractType
                 ->add('nationality', CountryType::class, $this->mergeFieldState([
                     'label' => 'Nationalität',
                     'property' => 'nationality',
+                    'required' => true,
                     'preferred_choices' => ['D', 'A', 'CH'],
+                    'empty_data' => 'D',
                 ], $getState('nationality')))
                 ->add('email', EmailType::class, $this->mergeFieldState([
                     'label' => 'E-Mail',