ConfigurationManager issue when using typemock

When i use typemock to isolate a static method, there is something wrong with ConfigurationManager AppSetting.
Here is my class:

public class TestTypeMock
    {
        public static readonly string IsTypeMockTest = ConfigurationManager.AppSettings["IsTypeMockTest"];

        public static void WriteTestMessage()
        {
            Console.WriteLine(ConfigurationManager.AppSettings["IsTypeMockTest"]);
        }
    }

Here is my test code:

[TestMethod]
        [Isolated]
        public void WriteTestMessageTest()
        {
            Isolate.WhenCalled(() => TestTypeMock.WriteTestMessage()).IgnoreCall();
            //Assert.IsNotNull(TestTypeMock.IsTypeMockTest);
            Assert.IsNotNull(ConfigurationManager.AppSettings["IsTypeMockTest"]);
        }

This case failed because the value is null, but when i uncomment the commented line, the case passed. Why I got this result? This error makes many of our test cases failed.