InvalidArgumentException: sink must not be a boolean

InvalidArgumentException: sink must not be a boolean in GuzzleHttp\Client->applyOptions() (line 384 of /var/www/html/vendor/guzzlehttp/guzzle/src/Client.php).

GuzzleHttp\Client->transfer(Object, Array) (Line: 125)
GuzzleHttp\Client->requestAsync('GET', Object, Array) (Line: 131)
GuzzleHttp\Client->request('GET', 'https://google-webfonts-helper.herokuapp.com/api/fonts/montserrat', Array) (Line: 72)
Drupal\google_webfonts_helper\RestApi->doRequest('/api/fonts/montserrat', Array) (Line: 100)
Drupal\google_webfonts_helper\RestApi->downloadFont('montserrat', Array, 'temporary://google-webfonts-helper/montserrat.zip') (Line: 45)
Drupal\google_webfonts_helper\FontDownloader->download('montserrat', Array, Array) (Line: 101)
Drupal\google_webfonts_helper\FontManager->download(Object) (Line: 83)
Drupal\google_webfonts_helper\FontManager->prepare('montserrat', ) (Line: 120)
Drupal\google_webfonts_helper\Entity\GoogleWebfont->validateFiles() (Line: 111)
Drupal\google_webfonts_helper\Entity\GoogleWebfont->postSave(Object, ) (Line: 534)
Drupal\Core\Entity\EntityStorageBase->doPostSave(Object, ) (Line: 460)
Drupal\Core\Entity\EntityStorageBase->save(Object) (Line: 263)
Drupal\Core\Config\Entity\ConfigEntityStorage->save(Object) (Line: 395)
Drupal\Core\Entity\EntityBase->save() (Line: 613)
Drupal\Core\Config\Entity\ConfigEntityBase->save() (Line: 323)
Drupal\Core\Entity\EntityForm->save(Array, Object) (Line: 245)
Drupal\google_webfonts_helper\Handler\Form\GoogleWebfontForm->save(Array, Object)
call_user_func_array(Array, Array) (Line: 112)
Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object) (Line: 52)
Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object) (Line: 591)
Drupal\Core\Form\FormBuilder->processForm('google_webfont_add_form', Array, Object) (Line: 320)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 91)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 573)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 151)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 694)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)