ASP.NET-API 400获取请求时出错

我有这个控制器:

namespace Cantrel.Application.CantrelSearchApi.Controllers
{
    [Route("api/[controller]")]
    [Authorize]
    public class MonitorsController : Controller
    {
        private readonly IMonitoringApiService monitoringService;
        private readonly IClientsApiService clientsService;
        private readonly ILogger<MonitorsController> logger;

        public MonitorsController(IMonitoringApiService monitoringService,
            IClientsApiService clientsService,
            ILogger<MonitorsController> logger)
        {
            this.monitoringService = monitoringService;
            this.clientsService = clientsService;
            this.logger = logger;
        }

  [HttpGet("{id}")]
    public async Task<IActionResult> Get(string id)
    {
        if (string.IsNullOrEmpty(null))
        {
            return BadRequest("No id was provided. Please provide a valid monitor id or subject id.");
        }

        try
        {
            MonitorDto monitor;

            if (Guid.TryParse(id, out Guid monitorId))
            {
                monitor = await GetByMonitorId(monitorId);
            }
            else
            {
                monitor = await GetBySubjectId(id);
            }

            if (monitor == null)
            {
                return NotFound();
            }

            return Json(monitor);
        }
        catch (Exception ex)
        {
            logger.LogError($"[{Request.Path.Value}]: {ex.ToString()}");
            return new StatusCodeResult(500);
        }
    }

我用邮递员打以下电话:

GET: {{api}}/Monitors/ABCDEFGH-1234-4567-8901-ABCDEFGHIJKL

我收到了一个 400 Bad Request 消息出错 No id was provided. Please provide a valid monitor id or subject id .

我不确定我是否明白这是怎么回事。字符串显然既不为空也不为空。

请告诉我是否有任何其他类别的代码,我应该提供帮助诊断。