MasterPageFile in web.config - Page Designer Errors

[Guest]

I have set the masterPageFile property in my web.config as follows:

<pages masterPageFile="~/Main.master" >

When I create a webform and I don't specify a masterPageFile in the aspx (so
that it uses the web.config specified master) and I go into design view for
the aspx, I get the following error:

Error Creating Control - Content
Content controls can only be used in a content page

Am I setting up the default master page incorrectly in the web.config file?

Thanks for your help