Possible bug?

Hi, first post here.

I'm trying to find my way around gh by trying out components. So I may be overlooking something basic. I do cnc programming by creating closed curves in rhino and then using rhinoCAM to create nc files. Point coordinates lists become polylines. I often add or subtract 3d points from each other. I tried to script a gh python component which replaces empty input with zero ie

If not x:
X= 0

If not y:
Y = 0

If not z:
Z = 0

And then the output streams are labelled X,Y and Z.

Y and Z work fine but X raises an error.

Thanks.