Full data path of the X resolution

I’m making a driver and I need to know the full path for the render x resolution value for the current scene.

The driver code is pretty easy:
full_path_to_x_resolution * 0.2 * another variable
However I can not guess the full path of the x resolution.
I’ve tried to put the mouse over the x resolution and press right button and then Copy Data Path, but I only get “render.resolution_x”.
What’s of the full name? Thanx

Wait… something strange is happening
I just tried bpy.context.scene.render.resolution_x in the Python console and it works!! The problem is it doesn’t work in the python expression line in the driver.
Even I just tried “bpy.context.scene.render.resolution_x” and still getting 0 in the debug value.
Ideas? Am I doing something wrong?
UPDATE:Solved
The problem was the build I was using. In an older build it works fine.