diff --git a/core/modules/block/lib/Drupal/block/BlockListController.php b/core/modules/block/lib/Drupal/block/BlockListController.php
index 874ff35..0a3285b 100644
--- a/core/modules/block/lib/Drupal/block/BlockListController.php
+++ b/core/modules/block/lib/Drupal/block/BlockListController.php
@@ -391,7 +391,10 @@ public function getOperations(EntityInterface $entity) {
    * Implements \Drupal\Core\Form\FormInterface::validateForm().
    */
   public function validateForm(array &$form, array &$form_state) {
-    // No validation.
+    $content = $form_state['values']['edited_theme'] . '_content';
+    if (isset($form_state['values']['blocks'][$content]['region']) && $form_state['values']['blocks'][$content]['region'] == -1) {
+      form_set_error('blocks][' . $content, t('Main page content block needs to be placed in some region.'));
+    }
   }
 
   /**