diff --git a/src/AtolOnline/Api/Kkt.php b/src/AtolOnline/Api/Kkt.php
index 979e464..ec2d872 100644
--- a/src/AtolOnline/Api/Kkt.php
+++ b/src/AtolOnline/Api/Kkt.php
@@ -118,7 +118,7 @@ class Kkt extends Client
         if (!$this->isTestMode()) {
             if (empty($login)) {
                 throw new AtolKktLoginEmptyException();
-            } elseif (strlen($login) > 100) {
+            } elseif ((function_exists('mb_strlen') ? mb_strlen($login) : strlen($login)) > 100) {
                 throw new AtolKktLoginTooLongException($login, 100);
             }
         }
diff --git a/src/AtolOnline/Entities/Client.php b/src/AtolOnline/Entities/Client.php
index b57fc69..141b765 100644
--- a/src/AtolOnline/Entities/Client.php
+++ b/src/AtolOnline/Entities/Client.php
@@ -89,7 +89,7 @@ class Client extends Entity
     public function setName(string $name)
     {
         $name = trim($name);
-        if (strlen($name) > 256) {
+        if ((function_exists('mb_strlen') ? mb_strlen($name) : strlen($name)) > 256) {
             throw new AtolNameTooLongException($name, 256);
         }
         $this->name = $name;
@@ -119,7 +119,7 @@ class Client extends Entity
     public function setPhone(string $phone)
     {
         $phone = preg_replace("/[^0-9+]/", '', $phone);
-        if (strlen($phone) > 64) {
+        if ((function_exists('mb_strlen') ? mb_strlen($phone) : strlen($phone)) > 64) {
             throw new AtolPhoneTooLongException($phone, 64);
         }
         $this->phone = $phone;
diff --git a/src/AtolOnline/Entities/Company.php b/src/AtolOnline/Entities/Company.php
index 900b036..289e706 100644
--- a/src/AtolOnline/Entities/Company.php
+++ b/src/AtolOnline/Entities/Company.php
@@ -115,7 +115,7 @@ class Company extends Entity
     public function setPaymentAddress(string $payment_address)
     {
         $payment_address = trim($payment_address);
-        if (strlen($payment_address) > 256) {
+        if ((function_exists('mb_strlen') ? mb_strlen($payment_address) : strlen($payment_address)) > 256) {
             throw new AtolPaymentAddressTooLongException($payment_address, 256);
         }
         $this->payment_address = $payment_address;
diff --git a/src/AtolOnline/Entities/Item.php b/src/AtolOnline/Entities/Item.php
index f421336..980659c 100644
--- a/src/AtolOnline/Entities/Item.php
+++ b/src/AtolOnline/Entities/Item.php
@@ -137,7 +137,7 @@ class Item extends Entity
     public function setName(string $name)
     {
         $name = trim($name);
-        if (strlen($name) > 128) {
+        if ((function_exists('mb_strlen') ? mb_strlen($name) : strlen($name)) > 128) {
             throw new AtolNameTooLongException($name, 128);
         }
         $this->name = $name;
@@ -328,7 +328,7 @@ class Item extends Entity
     public function setUserData(string $user_data)
     {
         $user_data = trim($user_data);
-        if (strlen($user_data) > 64) {
+        if ((function_exists('mb_strlen') ? mb_strlen($user_data) : strlen($user_data)) > 64) {
             throw new AtolUserdataTooLongException($user_data, 64);
         }
         $this->user_data = $user_data;
diff --git a/src/AtolOnline/Traits/HasEmail.php b/src/AtolOnline/Traits/HasEmail.php
index 3aa5343..c65f211 100644
--- a/src/AtolOnline/Traits/HasEmail.php
+++ b/src/AtolOnline/Traits/HasEmail.php
@@ -44,7 +44,7 @@ trait HasEmail
     public function setEmail(string $email)
     {
         $email = trim($email);
-        if (strlen($email) > 64) {
+        if ((function_exists('mb_strlen') ? mb_strlen($email) : strlen($email)) > 64) {
             throw new AtolEmailTooLongException($email, 64);
         }
         if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {