Error when creating new text format filter

When trying to create a new text format filter, get the following error:

Warning: Invalid argument supplied for foreach() in Drupal\Component\Utility\NestedArray::mergeDeepArray() (line 327 of core/lib/Drupal/Component/Utility/NestedArray.php).
Drupal\Component\Utility\NestedArray::mergeDeepArray(Array) (Line: 295)
Drupal\Component\Utility\NestedArray::mergeDeep(NULL, Array) (Line: 310)
Drupal\markdown\Form\SettingsForm->buildParser(Array, Object) (Line: 269)
Drupal\markdown\Form\SettingsForm->processSubform(Array, Object, Array)
call_user_func_array(Array, Array) (Line: 999)
Drupal\Core\Form\FormBuilder->doBuildForm('filter_format_add_form', Array, Object) (Line: 1062)
Drupal\Core\Form\FormBuilder->doBuildForm('filter_format_add_form', Array, Object) (Line: 1062)
Drupal\Core\Form\FormBuilder->doBuildForm('filter_format_add_form', Array, Object) (Line: 1062)
Drupal\Core\Form\FormBuilder->doBuildForm('filter_format_add_form', Array, Object) (Line: 563)
Drupal\Core\Form\FormBuilder->processForm('filter_format_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: 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: 708)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)