get_the_title() doesn't work in custom post type loop

I would like to display list of custom posts after single post. Is only one requirement: custom posts to display must have minimum one tag same as the single post title.

First, I have the loop – it’s working.
Rather then this code:

<?php

	$tag_slug = str_replace( ' ', '-', get_the_title() );
	$args = array(
		'post_type' => 'ksiazka',
		'tax_query' => array(
			array(
			'taxonomy' => 'tag_ksiazka',
			'field' => 'slug',
			'terms' => $tag_slug
			)
		),
		'post_status' => 'publish',
		'order'=> 'ASC',
		'orderby' => 'title'
		);

		$myposts = get_posts( $args );
		foreach ( $myposts as $post ) :
		setup_postdata( $post ); ?>

		<!-- post id, title, thumb, etc -->		

		<?php endforeach;
		wp_reset_postdata(); ?>

Everything is working properly when in place of ‘get_the_title()’ is real tag – posts list is correct.

Why ‘get_the_title()’ doesn’t working in this case?

Thanks for your help.