I wrote a CRC Xmodem code for C# and I don't know why it overflows

using System;

namespace CRC2
{
	class Program
	{
		public static ushort[] crc_table =
		{
	    	0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50a5, 0x60c6, 0x70e7,
	    	0x8108, 0x9129, 0xa14a, 0xb16b, 0xc18c, 0xd1ad, 0xe1ce, 0xf1ef
		};
		
		public static ushort CalculateCrc( byte[] data )
		{
		    int i;
		    ushort crc = 0;
		    int len = data.Length;
		
		    for (int j = 0; j < len; j++)
		    {
		        i = ( crc >> 12 ) ^ ( data[j] >> 4 );
		        crc = (ushort) (crc_table[ i & 0x0F ] ^ ( crc << 4 ));
		        i = ( crc >> 12 ) ^ ( data[j] >> 0 );
		        crc = (ushort) (crc_table[ i & 0x0F ] ^ ( crc << 4 ));
		    }
		
		    return crc;
		}
		
		public static void Main(string[] args)
		{
			string a = "1";
			
			byte [] numero = new byte[256];

			for (int i=0;i<a.length;i++)>
			{
				numero[i]=(byte)a[i];
			}
			
			ushort CRC = CalculateCrc(numero);


			Console.Write("CRC: ");
			Console.Write(CRC);
			
			Console.Write("Press any key to continue . . . ");
			Console.ReadKey(true);
		}
	}
}