diff --git a/src/Form/ParticipantType.php b/src/Form/ParticipantType.php
index ea288d9..ae6576c 100644
--- a/src/Form/ParticipantType.php
+++ b/src/Form/ParticipantType.php
@@ -323,9 +323,10 @@ class ParticipantType extends AbstractType
             }
 
             // forcibly select mandatory services that potentially have been disabled in PRE_SET_DATA
-            $mandatoryServices = array_filter($options['selectable_services'], function (Service $service) use ($data) {
+            $mandatoryServices = array_filter($options['selectable_services'], function (Service $service) use ($form) {
+                $participantIndex = $form->getData()->index;
                 return true === $service->mandatory ||
-                    (Service::SOURCE_BOOKING === $service->source && in_array($data['index'], $service->mapping));
+                    (Service::SOURCE_BOOKING === $service->source && in_array($participantIndex, $service->mapping));
             });
 
             if (0 < count($mandatoryServices)) {