custom_variable_classes in 9.1

I'm using some global variables through custom_variable_classes facility. I've recently switched from 8.1 to 9.1 

and somewhere along the line the behavior of custom_variable_classes has changed - if the variable has not been set 

for a given session invoking it (SELECT current_setting('name.variable')) throws and error whereas it used to return 'unset' value. 

Is there a way to control this behavior or maybe call it in a different way without an error ?

If not I'll have to think of setting this at each session start, which in turn has me asking: is there a way to call an SQL script

at each user login ?

pozdrowienia
mk