Problem for derivation of the fund using PLANT field

Hello.

I have a problem when I try do derivation rule based in PLANT field. The field is never filled. However, the correspondent field in BSEG table (WERKS) is filled in the process. The problem occurs in the TBB1 transaction for generate the accounting document in FI.

My derivation rule is based in this field and I don't understand why it is not filled.

Someone can help me?

Thanks.

Augusto Sabino