ASP.NET MVC - IHtmlFormElement's GetSubmission returns null

I'm making a POST request through an MVC controller using ASP.NET MVC and while unit testing I'm getting 'Object reference not set to an instance of an Object' from an IHtmlFormElement's GetSubmission() method. The test finds the button and the button itself is not null, but when the test calls the GetSubmission() with the submit button as a parameter it returns null.

Everything works fine from browser.

This is my Add.cshtml file content:

@model MVCPhones.Data.Phone

<form method="post" asp-action="Add">

    <div class="form-group">
        <label asp-for="Make" class="control-label">Make</label>
        <input asp-for="Make" type="text" class="form-control" />
    </div>
    <div class="form-group">
        <label asp-for="Model" class="control-label">Model</label>
        <input asp-for="Model" type="text" class="form-control" />
    </div>
    <div class="form-group">
        <label asp-for="RAM" class="control-label">RAM</label>
        <input asp-for="RAM" type="number" class="form-control" />
    </div>
    <div class="form-group">
        <label asp-for="PublishDate" class="control-label">PublishDate</label>
        <input asp-for="PublishDate" type="date" class="form-control" />
    </div>
    <button type="submit" class="btn btn-primary">Submit</button>
</form>

Here is my PhonesController post method:

        public async Task<IActionResult> Add(Phone phone)
        {
            try
            {
                phone.Created = DateTime.Now;
                phone.Modified = DateTime.Now;
                await _context.Phones.AddAsync(phone);
                await _context.SaveChangesAsync();

                return RedirectToAction(nameof(Index));
            }
            catch
            {
                return View();
            }
        }

Here is the section that gives the error: Error