diff --git a/CRM/Civicase/Hook/BuildForm/AddStyleFieldToCaseCustomGroups.php b/CRM/Civicase/Hook/BuildForm/AddStyleFieldToCaseCustomGroups.php
index dc44a29a3..c727f16b0 100644
--- a/CRM/Civicase/Hook/BuildForm/AddStyleFieldToCaseCustomGroups.php
+++ b/CRM/Civicase/Hook/BuildForm/AddStyleFieldToCaseCustomGroups.php
@@ -34,7 +34,7 @@ public function run($form, $formName) {
     // This is the generic entity for all cases.
     $caseEntityNames[] = 'Case';
 
-    $contactTypes = array_merge($contactTypes, $caseEntityNames);
+    $contactTypes = (is_array($contactTypes) && is_array($caseEntityNames)) ?  array_merge($contactTypes, $caseEntityNames) : [];
 
     $form->assign('contactTypes', json_encode($contactTypes));
     CRM_Core_Resources::singleton()->addSetting([