[support] str_replace in views argument

My second argument for a gmap style view is a city name, provided by url. It works fine for single word city names like 'Reseda' but not for multiple word names like 'Canoga Park'. The url supplies the name as 'Canoga_Park', with an underscore. I need to remove the underscore for the gmap lookup. I've set the default argument for Location:City to

return str_replace('_', ' ', $arg[2]);

No joy. I suspect this has something to do with the limitation that the argument return only a single value but I don't know enough about the underlying system to tell and I don't know how to debug a view.