Mysqli_fetch_assoc() expects parameter 1 to be mysqli_result

tolong dong bantu jawab, ini saya salah dimananya ya soalnya di error nya muncul nya gini : "Warning: mysqli_fetch_assoc() expects parameter 1 to be mysqli_result, bool given in D:\xampp\htdocs\bu wiwit\functions.php on line 9"

<?php
    $conn = mysqli_connect ("localhost", "root", "", "mahasiswa");
    $db = mysqli_select_db($conn,"mahasiswa");

    function query ($query){
        global $conn ;
        $result = mysqli_query($conn, $query);
        $rows = [];
        while ($row = mysqli_fetch_assoc($result)) {
            $rows[] = $row;
        }
        return $rows;
    }

?>

trus saya koneksi ke register.php saya dgn code sperti ini

<?php
    require "functions.php";
    $mahasiswa = query("SELECT * FROM mahasiswa");

    // if(isset($_POST["submit"])){
    //     var_dump($_POST);
    // }

?>

tolong bantu jawab yaa