Problem with XML port on import transactions

Hi, I have a problem during import a file with an XML Port.
I'm trying to import a text file where if the account number starts from B the Account Type should be bank.
On my xml -on Gen. Journal Line - Import::OnBeforeInsertRecord() i have the following line but the system prompts me an error when it reads the specific line because its searching the number from GL and not Banks.

IF COPYSTR("Gen. Journal Line"."Account No.",1,1) ='B' THEN BEGIN
AccountType := 3;

Can you please help me?