I can't figure out how to fix this error for my input file

 if( nextLine != null)
 {
     string[] nextItem = nextLine.Split(new char[] { ' ', '\t' }, StringSplitOptions.RemoveEmptyEntries);
     if (nextItem.Length == 0) { continue; }

     //BOXCAR
     if (nextItem[0].Equals("Box"))
     {
         //do something
         boxcar = Convert.ToInt32(nextItem[1], nextItem[2], nextItem[3]);
         writer.WriteLine(boxcar.Volume());
     }

     //TANKCAR
     if (nextItem[0].Equals("Tank"))
     {
         //do something
         tankcar = Convert.ToInt32(nextItem[1], nextItem[2]);
         writer.WriteLine(tankcar.Volume());
     }

     //REFRIGERATOR
     if (nextItem[0].Equals("Refrigerator"))
     {
         //do something
         refrig = Convert.ToInt32(nextItem[1], nextItem[2], nextItem[3], nextItem[4]);
         writer.WriteLine(refrig.Volume());
     }
 }