Update: I've now fixed this and it's been merged into Rails core, so use the master branch is you're having this issue.
If you have a non-nullable database column that you are using for the db backing of the new ActiveRecord Store in Rails 3.2, you will probably get an error such as:
TypeError: can't convert Symbol into Integer from .../activerecord-3.2.1/lib/active_record/store.rb:39:in `='
To fix it, change your column to :null => true
I've logged an issue report on github.