diff --git a/salsa_questionnaire/salsa_questionnaire.module b/salsa_questionnaire/salsa_questionnaire.module
index 8cfce9e..afbcac5 100644
--- a/salsa_questionnaire/salsa_questionnaire.module
+++ b/salsa_questionnaire/salsa_questionnaire.module
@@ -140,9 +140,11 @@ function salsa_questionnaire_form_submit($form, &$form_state) {
 
   // Then add supporter to the groups if any specified.
   $questionnaire = $form_state['salsa_object'];
-  $add_to_groups_KEYS = salsa_entity_string_explode($questionnaire->add_to_groups_KEYS);
-  $optionally_add_to_groups_KEYS = array_filter($form_state['values']['optionally_add_to_groups_KEYS']);
-  $groups_KEYS = array_unique(array_merge($add_to_groups_KEYS, $optionally_add_to_groups_KEYS));
+  $groups_KEYS = salsa_entity_string_explode($questionnaire->add_to_groups_KEYS);
+  if (!empty($form_state['values']['optionally_add_to_groups_KEYS'])) {
+    $optionally_add_to_groups_KEYS = array_filter($form_state['values']['optionally_add_to_groups_KEYS']);
+    $groups_KEYS = array_unique(array_merge($groups_KEYS, $optionally_add_to_groups_KEYS));
+  }
   if (!empty($groups_KEYS)) {
     foreach ($groups_KEYS as $key) {
       $supporter_groups[$key] = new SalsaEntity(array(), 'salsa_supporter_groups');