diff --git a/lib/RetailCrm/Methods/V5/Customers.php b/lib/RetailCrm/Methods/V5/Customers.php
index e4fb992..a9df396 100644
--- a/lib/RetailCrm/Methods/V5/Customers.php
+++ b/lib/RetailCrm/Methods/V5/Customers.php
@@ -164,9 +164,9 @@ trait Customers
             );
         }
 
-        if (!is_int($customerId)) {
+        if ($customerId === null || $customerId === '') {
             throw new \InvalidArgumentException(
-                'Parameter `customerId` must be an integer'
+                'Parameter `customerId` is empty'
             );
         }