after_update callback not acting as expected in Rails 3.2

I can't seem to get my callback to function as expected.

after_update :erase_reset_token, :on => :update, :if => :password

This calls a method on the User's model that should erase the password reset token from the database, but should only fire if it is the password that has been touched.

It's gotta be the syntax, but I can't figure it out. Any help, please?