Warning: Undefined array key "unknown-error" in recaptcha_captcha_validation()

Warning: Undefined array key "unknown-error" in recaptcha_captcha_validation() (line 241 of .../modules/contrib/recaptcha/recaptcha.module)

#0 .../core/includes/bootstrap.inc(164): _drupal_error_handler_real()
#1 .../modules/contrib/recaptcha/recaptcha.module(241): _drupal_error_handler()
#2 .../modules/contrib/captcha/captcha.module(522): recaptcha_captcha_validation()
#3 [internal function]: captcha_validate()
#4 .../core/lib/Drupal/Core/Form/FormValidator.php(282): call_user_func_array()
#5 .../core/lib/Drupal/Core/Form/FormValidator.php(238): Drupal\Core\Form\FormValidator->doValidateForm()
#6 .../core/lib/Drupal/Core/Form/FormValidator.php(118): Drupal\Core\Form\FormValidator->doValidateForm()
#7 .../core/lib/Drupal/Core/Form/FormBuilder.php(593): Drupal\Core\Form\FormValidator->validateForm()
#8 .../core/lib/Drupal/Core/Form/FormBuilder.php(325): Drupal\Core\Form\FormBuilder->processForm()
#9 .../core/lib/Drupal/Core/Entity/EntityFormBuilder.php(48): Drupal\Core\Form\FormBuilder->buildForm()
#10 .../modules/contact_block/src/Plugin/Block/ContactBlock.php(241): Drupal\Core\Entity\EntityFormBuilder->getForm()
#11 .../core/modules/block/src/BlockViewBuilder.php(171): Drupal\contact_block\Plugin\Block\ContactBlock->build()
#12 [internal function]: Drupal\block\BlockViewBuilder::preRender()
#13 .../core/lib/Drupal/Core/Security/DoTrustedCallbackTrait.php(111): call_user_func_array()
#14 .../core/lib/Drupal/Core/Render/Renderer.php(859): Drupal\Core\Render\Renderer->doTrustedCallback()
#15 .../core/lib/Drupal/Core/Render/Renderer.php(421): Drupal\Core\Render\Renderer->doCallback()
#16 .../core/lib/Drupal/Core/Render/Renderer.php(240): Drupal\Core\Render\Renderer->doRender()
.....

and shows log with
reCAPTCHA web service with empty message.