configSource only works on sections, not sectionGroups

I have an app.config with some custom sectionGroups:

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

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>.