Warning: array_flip(): Can only flip STRING and INTEGER values!

Dear Views Entity View Friends:

I am now consistently getting the following warning message after building a "views field view" that contains a field from a contextual view. The error shows up after I flush the cache and goes away when I refresh the browser:

Warning: array_flip(): Can only flip STRING and INTEGER values! in Drupal\Core\Entity\EntityStorageBase->loadMultiple() (line 227 of core\lib\Drupal\Core\Entity\EntityStorageBase.php).

Drupal\Core\Entity\EntityStorageBase->loadMultiple(Array) (Line: 57)
Drupal\node\Plugin\views\argument\Nid->titleQuery() (Line: 82)
Drupal\views\Plugin\views\argument\NumericArgument->title() (Line: 976)
Drupal\views\Plugin\views\argument\ArgumentPluginBase->getTitle() (Line: 1099)
Drupal\views\ViewExecutable->_buildArguments() (Line: 1254)
Drupal\views\ViewExecutable->build(NULL) (Line: 1378)
Drupal\views\ViewExecutable->execute() (Line: 263)
Drupal\views_field_view\Plugin\views\field\View->render(Object) (Line: 1144)
Drupal\views\Plugin\views\field\FieldPluginBase->advancedRender(Object) (Line: 224)
template_preprocess_views_view_field(Array, 'views_view_field', Array) (Line: 287)
Drupal\Core\Theme\ThemeManager->render(Array, Array) (Line: 435)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 195)
Drupal\Core\Render\Renderer->render(Array) (Line: 1736)
Drupal\views\Plugin\views\field\FieldPluginBase->theme(Object) (Line: 765)
Drupal\views\Plugin\views\style\StylePluginBase->elementPreRenderRow(Array)
call_user_func(Array, Array) (Line: 376)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 195)
Drupal\Core\Render\Renderer->render(Array) (Line: 713)
Drupal\views\Plugin\views\style\StylePluginBase->renderFields(Array) (Line: 580)
Drupal\views\Plugin\views\style\StylePluginBase->renderGrouping(Array, Array, 1) (Line: 467)
Drupal\views\Plugin\views\style\StylePluginBase->render(Array) (Line: 2117)
Drupal\views\Plugin\views\display\DisplayPluginBase->render() (Line: 1520)
Drupal\views\ViewExecutable->render() (Line: 171)
Drupal\views\Plugin\views\display\Page->execute() (Line: 1617)
Drupal\views\ViewExecutable->executeDisplay('purchase_contracts', Array) (Line: 78)
Drupal\views\Element\View::preRenderViewElement(Array)
call_user_func(Array, Array) (Line: 376)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 195)
Drupal\Core\Render\Renderer->render(Array, ) (Line: 226)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 574)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 227)
Drupal\Core\Render\MainContent\HtmlRenderer->prepare(Array, Object, Object) (Line: 117)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object, Object) (Line: 90)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object, 'kernel.view', Object) (Line: 111)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.view', Object) (Line: 149)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 64)
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: 99)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 78)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 50)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 652)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)