Error sending email in workflow - when using expression

Hi,

I have an email step in workflow. When I user recipient as 'User ID' and give a SAP User ID, mail is sent successfully. But when I use 'Expression', the mail is not sent. I have checked binding (workflow-> task, task->method) in the activity which determines user id and everything looks proper. Also in workflow log, I could see that user id is properly determined. But when I use the same expression in email step, I am getting following error:

Work item 000000482400: Object 000000482400 method SENDTASKDESCRIPTION cannot be executed

I have checked similar posts in SDN but could see that all those emails had attachments but my email does not have any attachment.

Please help me fix the issue.

Thanks & Regards

Vasu