The reason this isnt working is because the select list items dont have a numerical value. If you look at your source code you will probably see that the value is the same as the text and not a number.
To fix this you need to build the select list adding in the numerical values as well.
So you may need to build up your select list like this:
new SelectList(new[] {
new SelectListItem{Text = "something", Value = 0},
new SelectListItem{Text ="other", Value = 1},
new SelectListItem{Text ="third thing", Value = 2 }})