我搞错了

Fatal error: Uncaught Error: Call to a member function bind_param() on boolean

在这条线上：

$stmt->bind_param("i", $r);

我的问题准备得正确吗？我检查了表和列的名称，它们是正确的

$stmt = $conn->prepare("UPDATE db_control SET cve_usuario=? WHERE cve_control=1");
$stmt->bind_param("i", $r);

这是我的全部密码：

<?php
            $servername = "localhost";
            $username = "usuario";
            $password = "usuario";
            $database = "proyectofinal";
            // Create connection
            $conn = new mysqli($servername, $username, $password, $database);
            // Check connection
            if ($conn->connect_error) {
                die("Connection failed: " . $conn->connect_error);
            }
            $stmt = $conn->prepare("SELECT cve_usuario FROM db_control");
            if($stmt->execute())
            {
                $stmt->bind_result($r);
                if($stmt->fetch()){
                    echo $r;
                }
                $r = $r + 1;
                echo "<br>" . $r;
                $stmt = $conn->prepare("UPDATE db_control SET cve_usuario=? WHERE cve_control=1");
                $stmt->bind_param("i", $r);
                if($stmt->execute())
                {
                    /*do something*/
                }
            }
?>