DragPanelExtender BehaviorID Javascript Errors

In some javascript I have

var DragPanelExtender = ['<%=FormView1.FindControl("Panel1_DragPanelExtender").BehaviorID %>', "<%=Panel2_DragPanelExtender.BehaviorID%>"];

 I get the following error - Compiler Error Message: BC30456: 'BehaviorID' is not a member of 'System.Web.UI.Control'.

Any ideas??

Thanks