diff --git a/core/modules/book/src/BookStorageInterface.php b/core/modules/book/src/BookStorageInterface.php
index aa83c0d..ea03893 100644
--- a/core/modules/book/src/BookStorageInterface.php
+++ b/core/modules/book/src/BookStorageInterface.php
@@ -37,7 +37,7 @@ public function loadMultiple($nids);
    * @param array $book_link
    *   The book link.
    *
-   * @param int|string $max_depth
+   * @param int $max_depth
    *   The maximum supported depth of the book tree.
    *
    * @return int
@@ -48,7 +48,7 @@ public function getChildRelativeDepth($book_link, $max_depth);
   /**
    * Deletes a book entry.
    *
-   * @param int|string $nid
+   * @param int $nid
    *   Deletes a book entry.
    *
    * @return mixed
@@ -59,7 +59,7 @@ public function delete($nid);
   /**
    * Loads book's children using it's parent's id.
    *
-   * @param int|string $pid
+   * @param int $pid
    *   The parent's book nid.
    *
    * @return array
@@ -70,7 +70,7 @@ public function loadBookChildren($pid);
   /**
    * Builds tree data used for the menu tree.
    *
-   * @param int|string $bid
+   * @param int $bid
    *   The ID of the book that we are building the tree for.
    * @param array $parameters
    *   An associative array of build parameters. For info about individual
@@ -102,7 +102,7 @@ public function insert($link, $parents);
   /**
    * Updates book reference for links that were moved between books.
    *
-   * @param int|string $nid
+   * @param int $nid
    *   The nid of the book entry to be updated.
    * @param array $fields
    *   The array of fields to be updated.
@@ -115,7 +115,7 @@ public function update($nid, $fields);
   /**
    * Update the book id of the book link that it's beeing moved.
    *
-   * @param int|string $bid
+   * @param int $bid
    *   The ID of the book whose children we move.
    * @param array $original
    *   The original parent of the book link.