C# Displaying returned parameter as decimal within a label

I am using a returned parameter to display the weight of a bin.

Here is my SQL Server stored procedure;

ALTER PROCEDURE [dbo].[SumScrapBinWeight]
    @ScrapBinNumber INT,
    @ScrapBinWeightSUM DECIMAL(8,2) OUTPUT
AS 
BEGIN
    SELECT @ScrapBinWeightSUM = SUM(CalculatedWeightLbs) 
    FROM ScrapData 
    WHERE ScrapBinNumber = @ScrapBinNumber
END

This works in SSMS and returns the expected value, when I call this in my C# application I am getting an error

Cast not Valid

on the last line. This has me stumped, if I change Decimal to Int, it does not throw the error but it does not display the decimals I want to show.

using (SqlConnection conn = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["MfgDataCollector"].ToString()))
{
    DynamicParameters param = new DynamicParameters();
    param.Add("@ScrapBinNumber", lbl_TicketNumber.Text);
    param.Add("@ScrapBinWeightSUM", dbType: DbType.Int32, direction: ParameterDirection.Output);

    conn.Execute("SumScrapBinWeight", param, commandType: CommandType.StoredProcedure);

    lbl_BinWeight.Text = param.Get<decimal>("@ScrapBinWeightSUM").ToString();
}

Where am I going wrong here?