Masking a layer with another layer

I very often finding myself using a very handy masking option in the Layers palette. I create a shape in a layer, place it below the image I want to mask and then (holding down the option key) click the line between the layers and voila, the image is masked by the shape. It's a very useful little function. 

Today, however, I created a rounded corner box, added a gradient overlay to that box then tried to mask an image using my old trick. However, because the box has a gradient overlay the image disappears. I imagine the gradient overlay takes precedence over the image I want to mask. I almost feel dumb for asking but how can I most easily do this? Thanks!

Here is the image: