asp.net mvc - An MVC 6 route does not perform as expected -

i have route:

routes.maproute(name: "trads",     url: "test_3000/{action}/{traditional}",     defaults: new { controller = "test_3000", action = "subset", traditional = urlparameter.optional }); 

and test_3000controller method:

    // get: test_3000/subset?traditional=(chinese character) public actionresult subset(string traditional) {     if (traditional == null)     {         return new httpstatuscoderesult(httpstatuscode.badrequest);     }     test_3000 test_3000 = db.test_3000.find(traditional);     if (test_3000 == null)     {         return httpnotfound();     }     return view(test_3000); } 

this url works:

server/test_3000/subset?traditional=的

this url not work:

server/test_3000/subset/的

in latter case, 'traditional' null.

"traditional" column in sql table.

have tried this

routes.maproute(name: "trads",     url: "test_3000/{action}/{traditional?}",     defaults: new { controller = "test_3000", action = "subset", traditional = urlparameter.optional }); 

notice ? on traditional. also

public actionresult subset(string traditional  = null) { ... } 

so traditional explicitly set optional