Problem with double slash

Hello,

i’ve this problem.

In my functions.php file i’ve this code:

function add_a_slash( $permalink, $postID=”” ) {
return trailingslashit($permalink);
}
add_filter( ‘home_url’, ‘add_a_slash’);

Code that i need in order to have good canonical and href-lang url… but that code causes me this problem with cf7 (double slash …wp-json//contact-form-7…)

jquery-1.11.3.min.js:5 GET https://www.mywevsite.com/wp-json//contact-form-7/v1/contact-forms/1453/refill 404 ()

How could i fix it?

I could edit the code inside controller.php file? is it safe? there is another way to delete (only for this request) that double slash?

Thank you
Vincenzo