diff --git a/modules/market_management/service.php b/modules/market_management/service.php
index 6f727c0..7363a42 100644
--- a/modules/market_management/service.php
+++ b/modules/market_management/service.php
@@ -29,6 +29,15 @@ function market_management_require_positive_id(mixed $value, string $field): int
     return $id;
 }
 
+function market_management_optional_id(array $input, string $field): ?int
+{
+    if (!array_key_exists($field, $input) || $input[$field] === null || trim((string) $input[$field]) === '') {
+        return null;
+    }
+
+    return market_management_require_positive_id($input[$field], $field);
+}
+
 function market_management_optional_string(array $input, string $field): ?string
 {
     if (!array_key_exists($field, $input) || $input[$field] === null) {
@@ -433,6 +442,7 @@ function market_management_write_fields(array $input, array $definitions): array
             'number' => market_management_optional_number($input, $field),
             'integer' => market_management_optional_integer($input, $field),
             'id' => market_management_require_positive_id($input[$field], $field),
+            'nullable_id' => market_management_optional_id($input, $field),
             default => throw new LogicException('Unbekannter Feldtyp.'),
         };
         $fields[] = $field . ' = :' . $field;
@@ -566,7 +576,7 @@ function market_management_get_occurrence(PDO $pdo, int $occurrenceId, bool $inc
 function market_management_update_occurrence(PDO $pdo, int $occurrenceId, array $input): array
 {
     $definitions = [
-        'status' => 'string', 'place' => 'string', 'address' => 'string', 'municipality_reference_id' => 'id',
+        'status' => 'string', 'place' => 'string', 'address' => 'string', 'municipality_reference_id' => 'nullable_id',
         'notes' => 'string', 'registration_deadline' => 'string', 'request_or_registration_date' => 'string',
         'confirmation_date' => 'string', 'payment_deadline' => 'string', 'stand_size' => 'string',
         'table_available' => 'bool', 'table_rentable' => 'bool', 'own_table_required' => 'bool',