attempt to perform arithmetic on global 'cbtcur1' (a table value) help?

Title explains it all. I do not know how to solve the problem and the Roblox Forums didn't help, so here I am. Please help, because I need this ready for a friend by tomorrow.

cob=math.floor((#cbtcur1+#cbtcur2)/2)
    for i=1,cob do
        cb1=Instance.new("Part",SupportSeg)
        cb1.Name="CB1"
        if cbtcur1[i]<0 and cbtprev1[i]<0 then
        cb1.Size=Vector3.new(cbtcur1[i]-cbtprev1[i],.6,.6)
        cb1.Position=Vector3.new(cbtcur1-cbtprev1,cbtcury,cbtcurz)
        elseif cbtcur1[i]<0 and cbtprev1[i]<0 then
        cb1.Size=Vector3.new(cbtcur1[i]-cbtprev1[i],.6,.6)
        cb1.Position=Vector3.new(cbtcur1-cbtprev1,cbtcury,cbtcurz)
        else
        cb1.Size=Vector3.new(cbtcur1[i]+cbtprev1[i],.6,.6)
        cb1.Position=Vector3.new(cbtcur1+cbtprev1,cbtcury,cbtcurz)
        end