Volumetric light rendering a hard cut-off in the scatter rays I'm using Cycles in Blender 2.79

I have got a volume scatter material set up on a cube (0.2 density ) and a spot lamp (20000 intensity) above the volume (though I have the same problem if it's within the volume too) I'm getting a weird hard cut-off in the volumetric lighting and neither moving the lamp nor changing the intensity of the lamp changes where that hard cut-off is. 

The only thing that seems to change the height of the cut-off is moving the volume itself and it doesn't move by an equal amount. Any ideas what my trouble might be?


I figured out that the hard line is the edge of the volume out of camera clipping range. If I reduce the size of the volume then it renders correctly

Volume within camera clipping


Volume larger than camera clipping


I guess this is just how Blender works?
The problem was the camera clipping distance was less than the size of the volume.
By reducing the size of the volume so that the far side of the volume is within camera clipping distance the artifact (mostly disappeared)

A second problem I then had was the top of the volume I had given a surface shader (and a separate material) which caused lesser artifacts on top of any objects. Removing the surface shader fixed this.