Supplier Numbering Question

All,

We are trying to modify the setup for numbering of suppliers from manual alphanumeric to automatic alphanumeric

Setup --> Options --> Financials --> Supplier Number

Entry: Automatic

Type: Alphanumeric

Next Automatic Number: Im trying to give a value like D999, with the expectation that it will take. However, Im getting the error message as:

FRM-50016: Legal Characters are 0-9 - + E.

Any clue?

Thanks