diff -u b/src/Form/ServerForm.php b/src/Form/ServerForm.php
--- b/src/Form/ServerForm.php
+++ b/src/Form/ServerForm.php
@@ -107,7 +107,7 @@
       '#maxlength' => 50,
       '#required' => TRUE,
       '#machine_name' => [
-        'exists' => ['\Drupal\search_api\Entity\Server::load'],
+        'exists' => '\Drupal\search_api\Entity\Server::load',
         'source' => ['name'],
       ],
       '#disabled' => !$server->isNew(),