Coupling Constraint trouble

I read in the Abaqus documentation that a Coupling Constraint couples the motion of a surface to the motion of a single point.  I am modeling 1/8 of a square bar using symmetry wrt 3 planes at the origin (0,0,0). The free end of the bar is a surface perpendiclar to the X global axis. I define a ref point at (L,0,0,) where L is the length of the bar. Then I use the coupling constraint to `couple' the dof U1 at the surface at X=L to the ref point. I apply a single load CF1 at the ref point. The surface should move along X without rotating. It rotates !!! What have I done wrong ???