Get element based on string

I am creating a web api using mvc 6. now i am trying to get a element from my db. the key in this table is a string (email address). I do not have access to this database so i cant change the key of this table.

Now when creating a demo webapi i was able to create a controller to extract items based on a key which was an int. But when trying to get a element by a string the program crashes.

    [Route("api/[controller]")]
    public class TodoController : Controller
    {    

        [HttpGet("{id:string}", Name = "GetByIdRoute")]
        public IActionResult GetById (string id)
        {
            var item = _items.FirstOrDefault(x => x.Id == id);
            if (item == null)
            {
                return HttpNotFound();
            }

            return new ObjectResult(item);
        }
   }

when trying to access this path (example.com/api/Todo/key) key being the string i get an exception in my startup.cs

the exception in the browser reads:

System.InvalidOperationException The constraint entry 'id' - 'string' on the route 'api/Todo/{id:string}' could not be resolved by the constraint resolver of type 'DefaultInlineConstraintResolver'.

the part of the startup.cs where the code breaks is:

// Add MVC to the request pipeline.
            app.UseMvc(routes =>
            {
                routes.MapRoute(
                    name: "default",
                    template: "{controller}/{action}/{id?}",
                    defaults: new { controller = "Home", action = "Index" });   

            });

i cant seem to figure out why i ain't allowed to get a item by a key which is a string. is this even possible and if so what am i doing wrong?