Problem with texture node in Composite


I’m testing Blender composite nodes for some render effects. I followed this Pencil Sketch Rendering tutorial and basically it all went quite well. One little problem, though, and I just don’t find the answer:

The rendered image has one problem and it appears to be a generated texture that I use in the composite. The rendered result:

The problem is connected to a generated cloud texture (but also appears with other types of generated textures, not only clouds). Here are the composite nodes and the texture settings:

I tried to change any settings I could think of but to no avail. The ‘cut-off line’ sometimes moved up or down depending on the settings but it never disappeared completely. It appears as if the cloud texture has a specific height and just doesn’t cover the whole scene.

I’m just starting with Blender - if anyone has any hint why this happens I would really love to hear about it.