Fatal error: Call to a member function fetch_assoc() on a non-object in php

Question

I am getting error at fetch_assoc(). getTokenByEmail is a method which returns token.

<?php 
require_once 'DbOperation.php';
$db = new DbOperation();
$devices = $db->getTokenByEmail($_POST['u_code']);

$response = array(); 
$response['error'] = false; 
$response['devices'] = array(); 

while($device = $devices->fetch_assoc()){
$temp = array();
$temp['u_code']=$device['u_code'];
$temp['token']=$device['token'];
array_push($response['devices'],$temp);    
}

echo json_encode($response);
?>

This is the function getTokenByEmail().

public function getTokenByEmail($u_code){
    $stmt = $this->con->prepare("SELECT `token` FROM `user_reg` WHERE 'u_code'= ?");
    $stmt->bind_param("s",$u_code);
    $stmt->execute(); 
    $result = $stmt->get_result()->fetch_assoc();
    return array($result['token']);        
    }