Problem with DirectX Debug Mode

Hi! I try to make an application which uses Direct3D. It runs well, if the direct3D is in retail mode. But when I turn (in the control panel) the d3d to debug mode it generate strange errors. For example if i use viewport than always throw an error at the first device.Clear(..) funtion call. The type of error: InvalidCallException, Error in applicaion. If i don't set up a viewport it stops on an another function call. What is different in the debug mode or what do i wrong? Thank you in advance: Kemy