I can't add to local database, Failed to run query: SQLS

我正在尝试连接到本地主机，但不断得到错误声明:

Failed to run query: SQLSTATE[HY093]: Invalid parameter number:参数未定义

我得到这个错误后，我将我的详细信息添加到注册表单，并将其发布到index.php我看不出我的代码有什么问题，就我所能看到的参数都是正确的(显然不是虽然)

期望的行为是查询正确执行。

index . php

require("config.inc.php");
//if posted data is not empty
if (!empty($_POST)) {
    if (empty($_POST['username']) || empty($_POST['password'])) {

        $response["success"] = 0;
        $response["message"] = "Please Enter Both a Username and Password.";

        die(json_encode($response));
    }
    $query        = " SELECT 1 FROM users WHERE username = :username";
    $query_params = array(
        ':user' => $_POST['username']
    );
    try {
        $stmt   = $db->prepare($query);
        $result = $stmt->execute($query_params);
    }
    catch (PDOException $ex) {
        die("Failed to run query: " . $ex->getMessage());
        $response["success"] = 0;
        $response["message"] = "Database Error. Please Try Again!";
        die(json_encode($response));
    }

    $row = $stmt->fetch();
    if ($row) {
        die("This username is already in use");
        $response["success"] = 0;
        $response["message"] = "I'm sorry, this username is already in use";
        die(json_encode($response));
    }
    $query = "INSERT INTO users ( username, password ) VALUES ( :username, :password ) ";
    $query_params = array(
        ':username' => $_POST['username'],
        ':password' => $_POST['password']
    );
    try {
        $stmt   = $db->prepare($query);
        $result = $stmt->execute($query_params);
    }
    catch (PDOException $ex) {
        die("Failed to run query: " . $ex->getMessage());
        $response["success"] = 0;
        $response["message"] = "Database Error. Please Try Again!";
        die(json_encode($response));
    }
    $response["success"] = 1;
    $response["message"] = "Username Successfully Added!";
    echo json_encode($response);

} else {
?>
    <h1>Register</h1> 
    <form action="index.php" method="post"> 
        Username:<br /> 
        <input type="text" name="username" value="" /> 
        <br /><br /> 
        Password:<br /> 
        <input type="password" name="password" value="" /> 
        <br /><br /> 
        <input type="submit" value="Register New User" /> 
    </form>
    <?php
}