String Route Constraint

Hi. I have an ASP .Net Core 1.1 MVC web API. How can I have a string route constraint in a controller action?

I have the following two actions:

        / GET: api/Users/5
        [HttpGet("{id:int}")]
        [Authorize]
        public async Task<IActionResult> GetUser([FromRoute] int id)
        {
            if (!ModelState.IsValid)
                return BadRequest(ModelState);

            User user = await _context.User.SingleOrDefaultAsync(m => m.UserId == id);

            if (user == null)
                return NotFound();

            return Ok(user);
        }

        // GET: api/Users/abcde12345
        [HttpGet("{nameIdentifier:string}")]
        [Authorize]
        public async Task<IActionResult> GetUserByNameIdentifier([FromRoute] string nameIdentifier)
        {
            if (!ModelState.IsValid)
                return BadRequest(ModelState);

            User user = await _context.User.SingleOrDefaultAsync(m => m.NameIdentifier == nameIdentifier);

            if (user == null)
                return NotFound();

            return Ok(user);
        }

The first one works but the second doesn't - .Net doesn't like the "string" constraint. So basically, id I execute an HTTPGET request again:

http://mywebsite.com/api/users/5

it must execute the first action and if I request

http://mywebsite.com/api/users/abcde

it must execute the second one... Any ideas? Thanks...