Text fields not listed as source of Geocoded value from another field

I'm trying to follow the Capturing a collection of addresses recipe, but I run into an issue when creating the "Coded Location" CCK field for my content type. As instructed, I select the widget "Geocoded value from another field". On the field configuration page, want to select a previously-created CCK Text field called "Location", but it does not appear in the list. However, all of my CCK File->Image fields do show up in the list.

(I ended up installing the Postal module, and linking to that instead, but in my real-world application I want to map out bus stops, which don't have postal addresses.)