Setting a Focus on TextBox placed inside an updatepanel on page Load

hi frens 

I am trying to set the focus on the textbox control on the login page 

the login page is placed on the master page which is having scriptmanager 

the textbox control is place inside the updatepanel 

I am using this code to set the focus on the textbox on the page load 

 ScriptManager.GetCurrent(Page).SetFocus(txtUserName.ClientID)

but it is not working 

please help