diff --git a/lib/API/JSON/v1/Subscribers.php b/lib/API/JSON/v1/Subscribers.php
index f776d15cf6..4608b8663b 100644
--- a/lib/API/JSON/v1/Subscribers.php
+++ b/lib/API/JSON/v1/Subscribers.php
@@ -323,7 +323,7 @@ class Subscribers extends APIEndpoint {
         return $this->badRequest([
           APIError::BAD_REQUEST => WPFunctions::get()->__('Please regenerate the CAPTCHA.', 'mailpoet'),
         ]);
-      } elseif (!hash_equals(strtolower($data['captcha']), $captchaHash)) {
+      } elseif (!hash_equals(strtolower($data['captcha']), strtolower($captchaHash))) {
         $this->captchaSession->setCaptchaHash(null);
         $meta = [];
         $meta['refresh_captcha'] = true;
diff --git a/tests/integration/API/JSON/v1/SubscribersTest.php b/tests/integration/API/JSON/v1/SubscribersTest.php
index ba18ea8e4e..b8a8d71f41 100644
--- a/tests/integration/API/JSON/v1/SubscribersTest.php
+++ b/tests/integration/API/JSON/v1/SubscribersTest.php
@@ -555,7 +555,7 @@ class SubscribersTest extends \MailPoetTest {
     $subscriber->email = $email;
     $subscriber->countConfirmations = 1;
     $subscriber->save();
-    $captchaValue = 'ihg5w';
+    $captchaValue = 'ihG5W';
     $captchaSessionId = 'abcdfgh';
     $this->captchaSession->init($captchaSessionId);
     $this->captchaSession->setCaptchaHash($captchaValue);