Report server data source stored credentials won't work

Hi,

I'm trying to use stored credentials on a report server data source but when I test the connection I get an error: "Log on failed. Ensure the user name and password are correct." I'm trying to get the data source to connect to a SQL Server 2008 R2 instance.

The account I'm trying to use as stored credentials is a Windows account. I have tested the account against the SQL server instance by logging into SharePoint with that account and using Windows authentication on the data source. That way the data source works just fine. What am I missing here?