getting error Input string was not in a correct format.

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ASP.NET

C#3.5

SQL-Server-2008R2

hello friend i need one help
i have a three tier architecture
in BEL.cs

public string ID { get; set; }
public string Name { get; set;}
public decimal Salary{get;set;}

in BLL.cs

public string insertdata(BEL objinsertdata)
      {
          DLL obj = new DLL();
         return obj.insertdatadll(objinsertdata);

and in DLL.cs

public string insertdatadll(BEL obj)
       {
           SqlConnection con = new SqlConnection("Data Source=.;Initial Catalog=master;Integrated Security=True");
           con.Open();
           SqlCommand cmd = new SqlCommand("proapmc1", con);
           cmd.CommandType = CommandType.StoredProcedure;

           cmd.Parameters.Add(new SqlParameter("@ID", SqlDbType.VarChar, 22));
           cmd.Parameters["@ID"].Value = obj.ID;

           cmd.Parameters.Add(new SqlParameter("@Name", SqlDbType.VarChar, 44));
           cmd.Parameters["@Name"].Value = obj.Name;

          cmd.Parameters.Add(new SqlParameter("@Salary", SqlDbType.Decimal, 7));
          cmd.Parameters["@Salary"].Value = obj.Salary;

           string str = "";
           cmd.Parameters.Add(new SqlParameter("@Error", SqlDbType.VarChar, 34));
           cmd.Parameters["@Error"].Direction=ParameterDirection.Output;

           cmd.ExecuteNonQuery();
           str = (string)cmd.Parameters["@Error"].Value.ToString();

           return str;
       }

and in sql field i have taken decimal in salary , ID in varchar(22), and name also varchar(33)
but once i run this codes so i am getting a error in salary field "Input string was not in a correct format."
please tell me where i am wrong and what i am missing