I'd imagine that some partition of your multigrid doesn't intersect the isosurface at that value, leaving an empty field that could be handled incorrectly by some subsequent part of the net. Multigrids and Composite Fields require each member Field to carry the same type data, and since we may be changing the type of the MG by Replacing a different data component into "data", maybe the new type isn't correctly propagated to the member if the member was empty to begin with.
You might want to loop through the members of your multigrid to do the integration separately on each and sum the result. Do the members of your multigrid overlap? If so, there's nothing in there to prevent the double-counting of the overlap region. Greg
