## NURBS basis function

I have a problem calculating a basis function for a NURBS. I dont understand this formula http://libnurbs.sourceforge.net/nurbsintro.pdf numbered (6). It is a recursive formula wich accesses previous values of Nip(u). What I dont understand is how Ni+1,p-1(u) is possible, look for eg. if we have 5 control points and 3 degree or 4 order and if we are currently calculating a basis function for i=5 (5 control point) and p=2, than that term would be N5+1,2-1 wich is N6,1 but this doesnt exist since we dont have 6 control point so we cant calculate that data ??