"Uninitialized string offset: 0 in conditional_fields_evaluate_dependency()" when saving node form with conditional fields applied

I have a simple dependency set up between a boolean field (dependee) and a short text field (dependent). In the node form the relationship appears to work just fine (checking or unchecking the box shows/hides the dependent field, respectively). But when I save the form, I get a series of these errors:

Notice: Uninitialized string offset: 0 in conditional_fields_evaluate_dependency() (line 660 of modules/conditional_fields/conditional_fields.api.inc).
conditional_fields_evaluate_dependency('edit', Array, Array) (Line: 473)
conditional_fields_evaluate_dependencies(Array, Array, Object) (Line: 358)
conditional_fields_dependent_validate(Array, Object, Array)
call_user_func_array('conditional_fields_dependent_validate', Array) (Line: 277)
Drupal\Core\Form\FormValidator->doValidateForm(Array, Object) (Line: 235)
Drupal\Core\Form\FormValidator->doValidateForm(Array, Object, 'node_blog_post_edit_form') (Line: 119)
Drupal\Core\Form\FormValidator->validateForm('node_blog_post_edit_form', Array, Object) (Line: 571)
Drupal\Core\Form\FormBuilder->processForm('node_blog_post_edit_form', Array, Object) (Line: 314)
Drupal\Core\Form\FormBuilder->buildForm('node_blog_post_edit_form', Object) (Line: 74)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 574)
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}()
call_user_func_array(Object, Array) (Line: 144)
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)

Regarding the configuration...

- dependent field is "Visible"
- when the dependee "is Checked"
- interaction with other dependencies: "OR"
- all other options left untouched