Obtaining next number from a external range.

Hi Abapers,

I'm trying to assign a field a number based on a range (SNRO), but when I use the 'classic' function module 'NUMBER_GET_NEXT' what I get it's just an error message which reads that I should assign an internal range (for anybody info, is the RV_BELEG for a kind of sales orders.

Should I program a self-obtain function module for getting that next number, or may I use an standard module?

Best Regards.