Can’t get ScriptManager.RegisterStartupScript in WebControl nested in UpdatePanel to work

I had an issue using this in a user control (in a page this worked fine); the Button1 is inside an updatepanel, and the scriptmanager is on the usercontrol.

protected void Button1_Click(object sender, EventArgs e)  
{  
    string scriptstring = "alert('Welcome');";  
    ScriptManager.RegisterStartupScript(this, this.GetType(), "alertscript", scriptstring, true);  
}

Now it seems you have to be careful with the first two arguments, they need to reference your page, not your control

ScriptManager.RegisterStartupScript(this.Page, this.Page.GetType(), "alertscript", scriptstring, true);