Source

File: wp-includes/rest-api/endpoints/class-wp-rest-revisions-controller.php

protected function get_parent( $parent ) {
		$error = new WP_Error(
			'rest_post_invalid_parent',
			__( 'Invalid post parent ID.' ),
			array( 'status' => 404 )
		);
		if ( (int) $parent <= 0 ) {
			return $error;
		}

		$parent = get_post( (int) $parent );
		if ( empty( $parent ) || empty( $parent->ID ) || $this->parent_post_type !== $parent->post_type ) {
			return $error;
		}

		return $parent;
	}