Null Coalesce Operator and Undefined Indexes

Hello, I'm using PHP 7.1 now and just changed some code to use the Null Coalesce Operator, like so: $ua = sanitize($_SERVER['HTTP_USER_AGENT']) ?? null; $request = sanitize($_SERVER['REQUEST_URI']) ?? null; $protocol = sanitize($_SERVER['SERVER_PROTOCOL']) ?? null; $method = sanitize($_SERVER['REQUEST_METHOD']) ?? null; However, this morning I found this line in my error log: [25-Nov-2017 03:51:48] PHP Notice: Undefined index: HTTP_USER_AGENT in /home/user/public_html/scripts/index.php on line 45 This doesn't make sense to me. I thought this function was supposed to eliminate this warning. Is it because I've wrapped the first variable with a function? How/why is this happening? Thanks, Jenni