diff --git a/packages/block-serialization-default-parser/class-wp-block-parser.php b/packages/block-serialization-default-parser/class-wp-block-parser.php
index 40a56fadfe1b27..61f287b21fc4e8 100644
--- a/packages/block-serialization-default-parser/class-wp-block-parser.php
+++ b/packages/block-serialization-default-parser/class-wp-block-parser.php
@@ -397,3 +397,17 @@ public function add_block_from_stack( $end_offset = null ) {
 		$this->output[] = (array) $stack_top->block;
 	}
 }
+
+/**
+ * WP_Block_Parser_Block class.
+ *
+ * Required for backward compatibility in WordPress Core.
+ */
+require_once __DIR__ . '/class-wp-block-parser-block.php';
+
+/**
+ * WP_Block_Parser_Frame class.
+ *
+ * Required for backward compatibility in WordPress Core.
+ */
+require_once __DIR__ . '/class-wp-block-parser-frame.php';