BAPI_SALESDOCU_CREATEFROMDATA1 - Sales document type problem

I am using BAPI_SALESDOCU_CREATEFROMDATA1 but I am getting a runtime error on the header structure. It is saying that the Sales document type [FD, OR, etc...] does not exist. The problem is that I know it exists in the system. Any ideas what may be causing the trouble?

Davis