Where to get valid rates?

hi,
i'm running the Import Standard Purchase Orders after inserting records in the PO interface tables. i'm getting the error "Rate specified is invalid". i looked at this article . i tried to do as suggested and set the rate to null, along with rate_date and rate_type fields. this works only for one currency and not the others. i'm thinking that this might be the default currency.

so now i checked the gl_daily_rates table but i'm not sure how to validate the value of the rates in the po_headers_interace againts the gl_daily_rates table.