diff --git a/ajax/showfields.php b/ajax/showfields.php
index 2540f54eb..d514c0553 100644
--- a/ajax/showfields.php
+++ b/ajax/showfields.php
@@ -58,6 +58,9 @@
 try {
     $visibility = PluginFormcreatorFields::updateVisibility($_POST);
 } catch (Exception $e) {
+    echo json_encode([
+        'error' => $e->getMessage(),
+    ]);
     http_response_code(500);
     exit();
 }
diff --git a/inc/fields.class.php b/inc/fields.class.php
index fc1552de9..5a6be7253 100644
--- a/inc/fields.class.php
+++ b/inc/fields.class.php
@@ -140,7 +140,8 @@ public static function isVisible(PluginFormcreatorConditionnableInterface $item,
       /** @var CommonDBTM $item */
       $itemtype = get_class($item);
       $itemId = $item->getID();
-      if (!isset(self::$visibility[$itemtype][$itemId])) {
+
+      if (!isset(self::$visibility[$itemtype]) || !array_key_exists($itemId, self::$visibility[$itemtype])) {
          self::$visibility[$itemtype][$itemId] = null;
       } else if (self::$visibility[$itemtype][$itemId] !== null) {
          return self::$visibility[$itemtype][$itemId];