"Undefined array key 1 in template_preprocess_html()" on the 404 page when adding extensions to index.php

Problem/Motivation

When visiting the index.php with extra tacked on at the end, such as index.php.test, a warning is produced:

Warning: Undefined array key 1 in template_preprocess_html() (line 1312 of core/includes/theme.inc).
template_preprocess_html(Array, 'html', Array)
call_user_func_array('template_preprocess_html', Array) (Line: 261)
Drupal\Core\Theme\ThemeManager->render('html', Array) (Line: 491)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 248)
Drupal\Core\Render\Renderer->render(Array) (Line: 158)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 638)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 159)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object, Object) (Line: 90)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object, 'kernel.view', Object)
call_user_func(Array, Object, 'kernel.view', Object) (Line: 111)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch(Object, 'kernel.view') (Line: 186)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 2) (Line: 76)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 2, 1) (Line: 53)
Drupal\Core\StackMiddleware\Session->handle(Object, 2, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 2, 1) (Line: 28)
Drupal\Core\StackMiddleware\ContentLength->handle(Object, 2, 1) (Line: 32)
Drupal\big_pipe\StackMiddleware\ContentLength->handle(Object, 2, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 2, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 2, 1) (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 2, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 2, 1) (Line: 36)
Drupal\Core\StackMiddleware\AjaxPageState->handle(Object, 2, 1) (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle(Object, 2) (Line: 166)
Drupal\Core\EventSubscriber\DefaultExceptionHtmlSubscriber->makeSubrequest(Object, '/system/404', 404) (Line: 122)
Drupal\Core\EventSubscriber\DefaultExceptionHtmlSubscriber->on404(Object) (Line: 109)
Drupal\Core\EventSubscriber\HttpExceptionSubscriberBase->onException(Object, 'kernel.exception', Object)
call_user_func(Array, Object, 'kernel.exception', Object) (Line: 111)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch(Object, 'kernel.exception') (Line: 239)
Symfony\Component\HttpKernel\HttpKernel->handleThrowable(Object, Object, 1) (Line: 91)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 53)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 28)
Drupal\Core\StackMiddleware\ContentLength->handle(Object, 1, 1) (Line: 32)
Drupal\big_pipe\StackMiddleware\ContentLength->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: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 36)
Drupal\Core\StackMiddleware\AjaxPageState->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle(Object, 1, 1) (Line: 741)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)