Calling a javascript function disables calendar extender

[devquest123]

I have a script manager and calendar extender on my aspx page. When I call a javascript function from my code behind page, the calendar extender doesn't work
anymore. I call it like this:
ScriptManager.RegisterStartupScript(Page, this.GetType(), "javascriptfunction", "<script language=javascript>function();</script>", true);
OR
Page.ClientScript.RegisterStartupScript(this.GetType(), "javascriptfunction", "<script language=javascript>function();</script>", true);

Any ideas?

Thank you.