Text domain problems on default HTML 5 searchform code

I am getting this recommendation from Theme Check even though my searchform.php uses the same code from the Codex for the HTML 5 searchform:

RECOMMENDED: Text domain problems in searchform.php. You have not included a text domain!

Line 3: <span class='screen-reader-text'><?php echo _x( 'Search for:', 'label' ) ?></span>

Line 4: <input type='search' class='search-field' placeholder='<?php echo esc_attr_x( '', 'placeholder' ) ?>' value='<?php echo get_search_query() ?>' name='s' title='<?php echo esc_attr_x( 'Search for:', 'label' ) ?>' />

The searchform.php code:

<form role="search" method="get" class="search-form" action="<?php echo home_url( '/' ); ?>">
	<label>
		<span class="screen-reader-text"><?php echo _x( 'Search for:', 'label' ) ?></span>
		<input type="search" class="search-field" placeholder="<?php echo esc_attr_x( '', 'placeholder' ) ?>" value="<?php echo get_search_query() ?>" name="s" title="<?php echo esc_attr_x( 'Search for:', 'label' ) ?>" />
	</label>
	<input type="submit" class="search-submit" value="<?php echo esc_attr_x( 'Search', 'submit button' ) ?>" />
</form>

Am I doing something wrong?

https://wordpress.org/plugins/theme-check/