error when calling a javascript fonction from asp c# code

Hi,

I have an error when I excute my code below and I do not understand where it is coming from.

page ASPX:

<asp:Button ID="Bt_refresh" OnClick="Bt_refresh_Click" runat="server" Text="Resfresh" />

page ASP:

protected void Bt_refresh_Click(object sender, EventArgs e){           

System.Web.UI.ScriptManager.RegisterStartupScript(this, this.GetType(), "Test", "<script type='text/javascript'>switchTo(event,'mapserv.xml');</script>", false);          

}

page JavaScript:

function switchTo(event, dzi) {   

console.log("Debut switchTo");   

if (dzi) {       

console.log(dzi);       

viewer.openDzi(dzi);       

console.log("open dzi");   

} else {       

viewer.close();       

console.log("viewer closed");   

}}

and I get that error:

1. Uncaught TypeError: Cannot read property 'openDzi' of nullLoad_image.js:14
   1. switchToLoad_image.js:14
   2. (anonymous function)

   However my link is working when I use this method in my ASPX page:

<a href="#" onclick="switchTo(event,'map.xml');">map</a>