diff --git a/public/api/market-occurrence.php b/public/api/market-occurrence.php
index 0a29c63..8fc0bd5 100644
--- a/public/api/market-occurrence.php
+++ b/public/api/market-occurrence.php
@@ -64,7 +64,12 @@ if ($requestMethod === 'POST') {
         if ($contactFields !== []) {
             $submittedContactId = trim((string) ($fields['contact_person_id'] ?? ''));
             $contactId = (int) ($submittedContactId !== '' ? $submittedContactId : ($existingOccurrence['contact_person_id'] ?? 0));
-            update_contact_details($pdo, $contactId, $contactFields);
+            $hasContactValue = array_reduce($contactFields, static fn (bool $carry, mixed $value): bool => $carry || trim((string) $value) !== '', false);
+            if ($contactId > 0 && ($hasContactValue || $submittedContactId !== '')) {
+                update_contact_details($pdo, $contactId, $contactFields);
+            } elseif ($hasContactValue) {
+                throw new InvalidArgumentException('Für E-Mail oder Telefon muss eine Kontaktperson ausgewählt sein.');
+            }
         }
         $organizerFields = [];
         foreach (['organization_contact_id', 'contact_person_id'] as $organizerField) {