diff --git a/core/modules/block/block.admin.inc b/core/modules/block/block.admin.inc
index 2259176..0808ff1 100644
--- a/core/modules/block/block.admin.inc
+++ b/core/modules/block/block.admin.inc
@@ -290,15 +290,14 @@ function block_admin_edit($entity_id) {
  *
  * @param string $plugin_id
  *   The plugin ID for the block instance.
- * @param string $theme
- *   The name of the theme for the block instance.
  *
  * @see block_menu()
  * @see block_admin_block_delete_submit()
  */
-function block_admin_block_delete($form, &$form_state, $plugin_id, $theme) {
+function block_admin_block_delete($form, &$form_state, $plugin_id) {
   $block = entity_load('block', $plugin_id);
   $subject = $block->label();
+  list($theme) = explode('.', $plugin_id);
   $form['id'] = array('#type' => 'value', '#value' => $plugin_id);
   $form['theme'] = array('#type' => 'value', '#value' => $theme);
   $form['subject'] = array('#type' => 'value', '#value' => $subject);
diff --git a/core/modules/block/custom_block/lib/Drupal/custom_block/Plugin/block/block/CustomBlock.php b/core/modules/block/custom_block/lib/Drupal/custom_block/Plugin/block/block/CustomBlock.php
index 37b11d4..6f7be2e 100644
--- a/core/modules/block/custom_block/lib/Drupal/custom_block/Plugin/block/block/CustomBlock.php
+++ b/core/modules/block/custom_block/lib/Drupal/custom_block/Plugin/block/block/CustomBlock.php
@@ -75,8 +75,9 @@ public function blockForm($form, &$form_state) {
    * Overrides \Drupal\block\BlockBase::blockValidate().
    */
   public function blockValidate($form, &$form_state) {
+    list(, $bid) = explode(':', $form_state['entity']->get('plugin'));
     $custom_block_exists = (bool) db_query_range('SELECT 1 FROM {block_custom} WHERE bid <> :bid AND info = :info', 0, 1, array(
-      ':bid' => $form_state['values']['delta'],
+      ':bid' => $bid,
       ':info' => $form_state['values']['info'],
     ))->fetchField();
     if (empty($form_state['values']['info']) || $custom_block_exists) {