Hi Jade,
You have data types in the second arg to define_function_args when it should be the attribute names. It also looks like you need to add single quotes around the function name in this call, too.
select define_function_args('organization__set','...');
Also, is reg_number really a varchar?
BTW, using define_function_args, there's a way to specify default values for attributes by appending ";_the_default_value_" to the attribute(s). I'm not sure this is useful today, but it might be in the future ...