Problem with Custom Fields off Pages

Hello everybody.

I have the following problem. This is my current code I tried to use to show my custom field of a page:

if( is_single() || is_page() ) {

    global $post;
    $post_id = $post;

    if (is_object($post_id))
    $post_id = $post_id->ID;

    $pic = get_post_meta($post_id, '_pic', true);

    if ( isset($pic) && !empty($pic) && strstr($pic, "https://") )
echo $pic ';

but nothing happens… it works very well with posts but not with pages. can someone pls help me. thx a lot ??