Mocking ConfigurationManager.GetSection fails

Firstly, I searched the forum (and google) and found this. Sadly, none of the linked posts were helpful in solving my problem.

This is my test:

    [TestFixture]
    [Isolated]
    public class GetExternalEndpointsActivityFixture
    {
        [Test]
        public void Execute_WillReturnConfiguredExternalEndpoints()
        {
            Isolate.Fake.StaticMethods(typeof(ConfigurationManager));
            var mySection = Isolate.Fake.Instance<MyConfigSection>();
            var externalEndpointsCollection = Isolate.Fake.Instance<ExternalEndpointsCollection>();
            Isolate.WhenCalled(() => mySection.ExternalEndpoints = externalEndpointsCollection);
            var sectionName = "mySection";
            Isolate.WhenCalled(() => ConfigurationManager.GetSection(sectionName)).WillReturn(mySection); // <-- This fails
        }
    }

I've removed anything that isn't related directly to the problem. On the final line, which is marked with a comment, TypeMock complains:

TypeMock.TypeMockException :
*** No method calls found in recording block. Please check:
* Are you trying to fake a field instead of a property?
* Are you are trying to fake an unsupported mscorlib type? See supported types here: https://www.typemock.com/mscorlib-types

This is a method, not a field, so I assume it isn't talking about that. Also, ConfigurationManager isn't part of the mscorlib, and I understand that what I am doing should be fully supported by TM.

The version I am using is 7.0.6.0

Thanks,
Gary