select helper in Rails 3

Stupid one: anybody have problems with the select helper not selecting
an item?

There are no integer values for my options (they're all strings), so
it's not the usual integer/string conversion problem I've read about.
Imagine "Red," "Green," and "Blue." The value in the database is
"Green," but it isn't displaying the selection, only the first item
(Red).

I get as far as InstanceTag.to_select_tag and can see that "Green" is
set as the selected_value. It's someplace after that, apparently,
where things get messed up.

The name of the form element is correctly set to the model attribute,
such as shirt[color].

I'll keep debugging but am wondering if there's some trick here I'm
not aware of.