Error catching parent category

Hi, I´m using the following code to display a menu of the categories where my post is

$category = get_the_category(); 

			$parent = $category[0]->term_id;
			$category_link = get_category_link( $parent  );

			$args = array(
				'child_of' =>  $parent,
				'order' => 'desc',
			);

			$categories = get_categories( $args );

and it works fine but on some posts I have the following structure:

Cat 11 (parent)
Cat 13(child)

but when I echo the $parent it says that the 13 is the parent and it´s not displaying the menu but it only happens in some categories and I don´t know why is this happening