Steps to reproduce:
1. Open attached project "scrollRectAndMask"
2. Open "repro" scene
3. Navigate Canvas>Panel>GameObject in hierarchy
4. Add Mask component to "GameObject" object

Result: Mask doesn't work as it requires Image component.

Workaround: Use 2D Rect Mask instead of Mask

Note: I personally expected that if you add Mask, Image component is being added automatically. Also there is nothing mentioned about Image component in documentation:
http://docs.unity3d.com/Manual/script-Mask.html

Reproduced with: Unity 5.3.4p5; 5.4.0b16;