asp.net mvc - MVC RadioButtonFor default checked when enum but not for int in viewmodel -

noticed new thing today while trying solve problem have.

viewmodel:

[required] public paymentoption paymentoption { get; set; } 

enum:

public enum paymentoption {     creditcard,     invoice } 

view:

<div>     @html.radiobuttonfor(x => x.paymentoption, paymentoption.creditcard, new dictionary<string, object> {{ "id" , "cc" }}) credit card     @html.radiobuttonfor(x => x.paymentoption, paymentoption.invoice, new dictionary<string, object> {{ "id" , "in" }}) invoice </div> 

if changed paymentoption property int none of radiobuttons checked default when rendering view.

public int paymentoption { get; set; } 

if paymentoption property paymentoption enum 1 of radiobuttons checked default , in html source input radio has checked=checked.

public paymentoption paymentoption { get; set; } 

why behavior? shouldn't same both int , enum paymentoption property?