Assigning more than one Modifier Keys to a Key Gesture SHIFT + F not supported

I have the following code:

MyPlayPause.InputGestures.Add(new KeyGesture(Key.P, ModifierKeys.Control));

I need to add another gesure SO I CAN HAVE SHIFT + CTRL + P but it breaks when i add the option for:

MyPlayPause.InputGestures.Add(new KeyGesture(Key.P, ModifierKeys.Control));

shift option. I get this error: 'Shift+F' key and modifier combination is not supported for KeyGesture.

Any idea why? I need to replicate the functionality of the Media Player fast forward button.