Why don't work?

var count = 3;
var fakeCollection1 = Isolate.Fake.Instance<ICollection>();
var fakeCollection2 = Isolate.Fake.Instance<ICollection>();
Isolate.WhenCalled(() => fakeCollection1.Count).DoInstead(context => fakeCollection2.Count);
Isolate.WhenCalled(() => fakeCollection2.Count).WillReturn(count);
Assert.AreEqual(fakeCollection1.Count, count);