add exception for Profile

Any1 know how 2 add error mesage for the Profile?
<script runat="server">
    Protected Sub CreateUserWizard1_CreateUser(ByVal sender As Object, _
ByVal e As System.EventArgs)
        Dim pc As ProfileCommon = New ProfileCommon()
        pc.Initialize(CreateUserWizard1.UserName.ToString(), True)
        pc.StalkNumber = CType(CreateUserWizard1.CreateUserStep.ContentTemplateContainer.FindControl("StalkNumber"), TextBox).Text
        pc.StalkName = CType(CreateUserWizard1.CreateUserStep.ContentTemplateContainer.FindControl("StalkName"), TextBox).Text
        pc.Save()
    End Sub
</script>
The data will not be insert in the database if there are same StalkNumber but now I have no idea ...