I have imported an MH-114 airfoil into solid works as shown in the picture.
 

The trailing edge of this airfoil was open (typical of airfoil coordinates) so I closed it using tangent lines at the trailing edge making a closed contour. The whole in the middle is for spar. Then I extruded it to give the following geometry.
 

Now the problem is, i would like to offset the outer surface of this airfoil. The offset surface command works for small offsets when offsetting outwards, but when offsetting inwards, I only get a solution when I use a value of 1mm. I get error on a value smaller and bigger then this. What could be the problem. 
Here is the part file: