Assign external # range to Requisition using BAPI_REQUISITION_CREATE

I am using BAPI_REQUISITION_CREATE to create a Requisition with a specified number. The number is in the External Number range of the document type. When I create the Requisition in transaction ME51N I can assign the number I require. However, when I try to assign the same number range using the BAPI it says the number is not in the number range.

I am converting the number using the ALPHA convert function, but the BAPI still complains that the number is not in range.

Does anyone know if I need to do something in the BAPI settings to get it to accept the External Number range for the document type.

Thank you

Karen