Depreciation-External Number Range

Hi Sap Guru's,

Actually for Depreciation-Document Type AF,,we have to maintain number range With EXTERNAL Check box activation....

But here in my company(currency type-USD),,,while running the depreciation it is giving the error like"REMOVE THE CHECK BOX FOR EXTERNAL NUMBER RANGE-DOCUMENT TYPE AF)..

what is the reason?????can we do without external number range activation????

give me reply

regards

Sap Guru