Cannot get a response from a Web API call

I have the following in the TreeController controller in a small web API:

    [HttpGet("GetDirectories")]
    public IActionResult GetDirectories()
    {
        var baseDir = _config["QuickShare:BaseDir"];
        if (string.IsNullOrWhiteSpace(baseDir))
        {
            throw new InvalidOperationException("'QuickShare:BaseDir' is not configured");
        }
        var ret = GetDirectories(baseDir); ;
        return Json(ret);
    }
    private List<DirectoryInfo> GetDirectories(string parentDir)
    {
        var dirInfo = new DirectoryInfo(parentDir);
        return dirInfo.GetDirectories("*", SearchOption.TopDirectoryOnly).ToList();
    }

When I try and call this action from Postman, I get told

Could not get any response There was an error connecting to http://localhost:59243/api/Tree/GetDirectories.

Now the default, test, controller that comes with the project template is unchanged:

[Route("api/[controller]")]
public class ValuesController : Controller
{
    // GET api/values
    [HttpGet]
    public IEnumerable<string> Get()
    {
        return new string[] {"value1", "value2"};
    }
    ...
}

And when I have the API running in Visual Studio, I can get a proper response from the Values controller, but not from the TreeController, yet they are almost exactly the same. And, when I call the Tree/GetDirectories` action, a breakpoint in that action method is hit, and I can single step through the very few lines that my method has, and they all execute fine.

The problem only becomes apparent when that last line of the action executes:

return Json(ret);

Then I get shown that Postman Could not get any response despite no exception being raised; while debugging the code, it looks like everything should work fine, and the requests to the Values controller work fine.

Your action method should return either the specific result type (JsonResult when you return Json ) or IActionResult.

Example:

 [HttpGet("GetDirectories")]
    public JsonResult GetDirectories()
    {
        var baseDir = _config["QuickShare:BaseDir"];
        if (string.IsNullOrWhiteSpace(baseDir))
        {
            throw new InvalidOperationException("'QuickShare:BaseDir' is not configured");
        }
        var ret = GetDirectories(baseDir);
        return Json(ret);
    }

OR

[HttpGet("GetDirectories")]
public IActionResult GetDirectories()
{
    var baseDir = _config["QuickShare:BaseDir"];
    if (string.IsNullOrWhiteSpace(baseDir))
    {
        throw new InvalidOperationException("'QuickShare:BaseDir' is not configured");
    }
    var ret = GetDirectories(baseDir) ;
    return Ok(ret);
}

You can get more help from Microsoft Documentation: Formatting Response Data