Microsoft JScript runtime error: 'MyWebService' is undefined.

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I've created a web service which I'm just running on my localhost development
machine right now. If I open http://localhost/WebServices/MyWebService.asmx
in IE everything works fine. However when I use it in a web project where it
is declared in the ScriptManager section and then called in JavaScript it
seems that it can't find it.

In the aspx file within the <Servicessection of the <ScriptManagerI have:

<asp:ServiceReference Path="http://localhost/WebServices/MyWebService.asmx" />

and in the JavaScript, which is where it crashes, I have

ret = MyWebService.ServiceMyRequest();
Any ideas?