diff --git a/src/AmoCRM/Models/ContactModel.php b/src/AmoCRM/Models/ContactModel.php
index ac26a319..cfad23f8 100644
--- a/src/AmoCRM/Models/ContactModel.php
+++ b/src/AmoCRM/Models/ContactModel.php
@@ -639,7 +639,7 @@ public function toApi(?string $requestId = "0"): array
         }
 
         if (!is_null($this->getLastName())) {
-            $result['first_name'] = $this->getLastName();
+            $result['last_name'] = $this->getLastName();
         }
 
         if (!is_null($this->getResponsibleUserId())) {