FieldMissingException at fakes' private fields accessors

Hello,

I have certain issue when I try to access private field using generated accessors and faked object. Non public accessor internally uses PrivateObject class and my test below is base on it.

I have an abstract class which implement some functionality for all future parents. The example of the class below:

public abstract class TestPrivate
{
    private string testField;

    //functionality in base class which should be tested 
    public virtual string DoWorkInBaseClass()
    {
        return testField;
    }
}

Then I tried to test this functionality without any concrete class implementation. I wrote the following test:

[TestMethod]
[Isolated]
public void IsolatedPrivateFieldFailureTest()
{
    string expectedValue = "test string to private fields";
    TestPrivate obj = Isolate.Fake.Instance<TestPrivate>(Members.CallOriginal);
    PrivateObject privateObj = new PrivateObject(obj);
    privateObj.SetField("testField", expectedValue);

    //Test some functionality in base class
    string actual = obj.DoWorkInBaseClass();

    Assert.AreEqual(expectedValue, actual);
}

This test fails with FieldMissingException. In real tests I uses generated private fields accessors which internally use PrivateObject class and fails with the same error.

BTW: if I add to the TestPrivate class abstract modifier the test above passes (but in this case Isolate framework just creates a real instance of TestPrivate class).

Do you have any ideas how can write my test with Isolate in this case?

TIA,
Grammer