Invoke private method with parameter having null value

Hi,
I tried to invoke a private method with parameter having value null using Arrange-Act-Assert style, but Typemock throws an expection; so I couldn't achieve my aim.
Here is my sample class:
public Class1
{
private int value;

private void MyPrivateMethod(bool? boolValue)
{
if(boolValue.HasValue)
{
if(boolValue.Value)
{
value = 1;
}
else
{
value = -1;
}
}
else
{
value = 0;
}
}
}
----------------------------------------------------------------------
Test code:

Class1 c1= new Class1();
Isolate.Invoke.Method(c1, "MyPrivateMethod", null);
-----------------------------------------------------------------------
I tried to make a mock instance of Class1 and invoke method on it, but I again received an exception. Please lead me in a way to achieve this.
Thanks.