arithmetic overflow error for data type tinyint,value = 256

byte number = 1; add(number); // form .cs

public static int Add( byte? order)
{
  arParams[0] = new SqlParameter("@number", (number.HasValue) ? ((object)number) : DBNull.Value);
  // stored procedure call is made which takes paramaters, 
}

Stored procedure looks like this

@number tinyint
AS
BEGIN
IF @number IS NOT NULL
BEGIN
  UPDATE 
    table1
  SET 
    number = number + 1
  WHERE 
    id=13
END
INSERT INTO 
 table1
(
  number
)
VALUES 
( 
  number=@number
)

///////why i am getting this error can any one illustrate please and how do i solve this