Workflow that sends an email when the value of the web address field is changed.

The workflow sends the email when I use a custom field on the customer record, with the following parameters:

Workflow Summary: Trigger on - Before Record Submit

State 1
Set Field Value, Condition: Old record: Web Address != Web Address, Trigger on Before Record Submit, Set "Send Email"=T

Send Email, Condition: "Send Email"=T, Trigger on After Record Submit

This works great. Now, if instead of using a custom field on the customer record, I use an instance custom field within the workflow, the email never gets triggered. The Send Email instance custom field is set to T in the first step, but then when it's evaluated for sending the email, it's evaluated to False.