NadconTransform

I have some coordinates in the state plane system, using the NAD83 datum, and I'd like to convert them to NAD27 using the NADCONTransform class. I've tried having geotools convert them into geographical coordinates (from lambert conic) but the NADCONTransform class keeps telling me that my points are not on the grid.

I'm starting from EPSG:2229 and converting to EPSG:4957 before passing my points off to the NADCONTransform, using conus.las/los for my grid. Is there something else I should be doing? Does anyone know how the NADCONTransform class is properly used?