post a bill to account payable, transaction balance is 0.00

[zhangweiwu at realss.com]

Hello. I am trying to manage account payable. I made a bill that has
only one invoice entry of 348$ (Online service as "expense account"),
and I post it to the 'account payable' I created under liability.

I expect to see the balance of "account payable" account change by 348
and so does "online service" account under "expense" account. However a
new transaction appear in "account payable" with balance of 0.00 and
both accounts balance remain 0.00.

What did I do wrong? Thanks in advance!