How to prevent XSS on the below code

How would I properly prevent Cross Site Scripting on the below code below? Do I need to do something with (Container is...)?  I have it on Username, so I am trying to find out why Fortify software would blow up on this.
 
Text='<%# (Container is IGridInsertItem) ? string.Empty : Microsoft.Security.Application.Encoder.HtmlEncode((string)Eval("Username")) %>'>