diff --git a/core/modules/book/book.module b/core/modules/book/book.module
index 6590aa8..6d0cac8 100644
--- a/core/modules/book/book.module
+++ b/core/modules/book/book.module
@@ -306,7 +306,7 @@ function book_block_view($delta = '') {
       }
     }
     if ($block['content'] = $book_menus) {
-      $book_menus['#theme'] = 'book_all_books_block';
+      $block['content']['#theme'] = 'book_all_books_block';
     }
   }
   elseif ($current_bid) {