I have been trying to generate polars for the tailfeathers of most small RC planes that are simply flat slabs. I programmed some airfoil coordinants into XFoil (Profili) with rounded edges and a 6% thickness and 0 camber to represent a flat balsa stab. When processed I get an error stating that 0 of 74 coordinants are present and increasing the FLIT and smoothing does not solve the problem. Is this an Xfoil problem or is there convergence or separation issues.

Can anyone make any suggestions?