Function and Local variable doesn't work inside another function - php

Question

Something is really strange or am I doing something wrong, I can't figure it out. When I add the variable $submenu in the wp_nav_menu args it doesn't work. See the examples below.
Error Message
Strict Standards: Only variables should be passed by reference in
This Works
$args = array(
'menu_id' => 'sidebar-menu',
'theme_location' => 'primary',
'submenu' => 'About Us'
);
wp_nav_menu( $args );
This doesn't work
$args = array(
'menu_id' => 'sidebar-menu',
'theme_location' => 'primary',
'submenu' => get_top_level_title()
);
wp_nav_menu( $args );
This function gets the top most parent title nothing fancy.
function get_top_level_title(){
global $post;
$category = get_the_category( $post->ID );
$parent_title = get_the_title( navz_top_most_parent_page_id() );
if( is_single() ){
$category = get_the_category( $post->ID );
$post->post_parent = $category[0]->term_id;
return get_cat_name($category[0]->category_parent);
} else if( $parent_title and !is_category() ){
return $parent_title;
} else {
$cat_obj = get_category( get_query_var( 'cat' ) );
$parent = $cat_obj->parent;
return get_cat_name($parent);
}
}
This code gets the function above into a local variable and passess it to the wp_nav_menu to get the submenu.
$submenu = get_top_level_title();
$args = array(
'menu_id' => 'sidebar-menu',
'theme_location' => 'primary',
'submenu' => $submenu
);
wp_nav_menu( $args );