Gradient tool does not seem to do anything

[onepath]

When I try to add a gradient over a mask by using the gradient tool, nothing happens. As shown in the attached image, I have a yellow rectangle with a mask, where I want to add the gradient over. However, when I use the gradient tool, and create the line over the rectangle in question, literally nothing happens.. It does not matter which gradient type I choose, nothing will happen. 

[davescm]

Choose a black to white gradient when applying to the mask

Dave