ModelState.IsValid threw an exception of type System.IO.FileLoadException

I have an .NET MVC application and I'm writing some unit test for it using NUnit.

This is the function I'm currently testing:

[HttpPost]
[ValidateAntiForgeryToken]
public async Task<ActionResult> Add(AddInstrumentViewModel model)
{
    if (ModelState.IsValid)
    {
        var group = new FtpServerGroup
        {
            Disabled = true,
            FtpLastPullTime = DateTime.Now,
            FtpPullInterval = TimeSpan.Zero,
        };

        await db.CreateAsync(group, model.Address, model.Username, model.Password);

        // go back to card view
        var res = await db.GetAll();
        return RedirectToAction("Index", res);
    }

    return View(model);
}

this is the actual test:

[Test]
public async Task Test()
{
    var controller = new AutomaticTransferController(mockService.Object);

    var model = new AddInstrumentViewModel
    {
        Address = "124.124.124.124",
        Password = "123456",
        Username = "John Foo"
    };

    var res = await controller.Add(model);
}

Stepping with the debugger I noticed that as soon as I enter the .Add() method the line if (ModelState.IsValid) throw an exception

ModelState.IsValid threw an exception of type System.IO.FileLoadException

In fact, it threw an exception without telling me when nor where