The name 'UserName' does not exist in the current context error

hi
i have TextBox inside  <asp:CreateUserWizard \>  -->  <WizardSteps>     -->   <asp:CreateUserWizardStep\>     -->     <ContentTemplate>
this TextBox ID is UserName.
I'm facing a problem to push into this TextBox. when i do 
UserName.Text = "some name";  i get this error : "The name 'UserName' does not exist in the current context"
any help?
thankes
protected void Page_Load(object sender, EventArgs e) { if (Session["VolunteerSessionList"] != null) // test if exists { UserName.Text = "some name"; }


<asp:CreateUserWizard ID="RegisterUs ...