resampling/evaluating 3D field of manifold dimension 2

I have a Gridded3DSet with a manifold dimension of 2:
   Set xyzSet = new Gridded3DSet( domainTuple, xyz,
                                       num_radials, num_gates );

I have a line in 3D:
    Set lineDomainSet = new Gridded3DSet( lineDomain, 
                                                 domainSamples, pts);

and I want to find the values of the first 3D set at samples
given by the second.  There are places where the line
does not intersect the surface, so I will get NaN in some spots, but
there will be points of interest as well.

So, I try to do:
    xyzSet.resample( lineDomainSet );

but I get an exception:
visad.SetException: Gridded3DSet.valueToGrid: ManifoldDimension must 
be 3   

Should I not be able to do this?

Lak

p.s. I can't evaluate the set at a point either:
 xyzSet.evaluate( (RealTuple) cursorDataRef[0].getData(), 
              Data.NEAREST_NEIGHBOR, Data.NO_ERRORS) );