calling membership user - to get email but getting error??

hi,
i have a datalist bound to SQL table - one of the columns is UserId.

In the datalist item template - there is a label to get UserId value and set to Visible = false.

In the code behind i call Membership.User to get the email address and use in a mailmessage
but am getting error and not sure why?? all the code is below
Object reference not set to an instance of an object. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. Exception Details: System.NullReferenceE ...