[netcdf-java] Underscaore as FillValue

I have been mostly successful in getting some really badly-formed netCDF into CF-compliant netCDF, but I have what seems to be one stumbling block left. The original netCDF has a FillValue specified of -30000., but the variable is defined as a float. When I try to change the _FillValue attribute to -30000.f in my CoordSysBuilder all of the data points that were previously -30000 become underscore characters. 

Anyone have an idea as to what's happening?

Thanks

Matt

--
Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. 
-- Albert Einstein


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