Update constant smart service Invalid Cast: Cannot cast from type 'Document' to type 'Content Constant'

As David indicated - for the smart service to work, you have to select the constant from the picker.  I'm not sure if there's any way to do it via the expressionable editor (which is what you probably did) which would work.  The problem this creates is, when the process goes to update the constant, instead of seeing a *reference to the constant* (which you'd get from the constant picker), instead it's evaluating the constant and returning its value - hence your error message.