Error occur during generating CNgrid in HECgeoHMS.

hi, I am bhavna using ArcMap9.3 with Windows 7 home basic 64 bit. I installed HEC-GeoHMS 5.0, ArcHydro tool9 for watershed modelling. when i tried to generate CNGrid (input fil, Lulc_soil_union and CNLookup table) from utilities of HEC-GeoHMS, its doesnot generating CNGrid (Grid formate) and showing the massage below
" System.NullReferenceException: Object reference not set to an instance of an object.
at ESRI.APWR.HECGeoHMS9.GPGenerateCNGrid.GenerateCNGridWk(ApLayers apLayers, Int32 debug, ITrackCancel& trackcancel, IGPMessages& messages, String& exMessage)
Failed to execute (GenerateCNGrid)."


bhavnapardhi90@gmail.com