configSource only works on sections, not sectionGroups

I have an app.config with some custom sectionGroups:

<configSections>
    <sectionGroup name="MyApp">
            <section name="foo"
                     type="System.Configuration.NameValueSectionHandler, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
    </sectionGroup>
</configSections>
<myApp>
    <foo>
        <add key="MySetting" value="14"></add>
    </foo>
</myApp>

I wanted to externalize that:

<MyApp configSource="myapp.config">

This yields an error:

System.Configuration.ConfigurationErrorsException: The attribute 'configSource' cannot be specified because its name starts with the reserved prefix 'config' or 'lock'.

Long story short: configSource only works on <section> elements, not on <sectionGroup>.