Не удается преобразовать значение параметра 'brandOfCar' из 'System.String' в 'AutoCatalog.InsertDetails'

public List<InsertDetails> GetModels_DDL(InsertDetails brandOfCar)
            {
                SqlConnection connection = Connection.Create();
                SqlCommand cmd = new SqlCommand("GetModels_DDL", connection);
                cmd.CommandType = CommandType.StoredProcedure;
                ListModels = new List<InsertDetails>();

                cmd.Parameters.Add(new SqlParameter("brandOfCar", System.Data.SqlDbType.NVarChar, 30));
                cmd.Parameters["brandOfCar"].Value = brandOfCar.BrandOfCar;

                try
                {
                    connection.Open();
                    SqlDataReader reader = cmd.ExecuteReader();
                    while (reader.Read())
                    {
                        InsertDetails insertDetails = new InsertDetails((string)reader["nameModel"]);
                        ListModels.Add(insertDetails);
                    }

                    reader.Close();
                    return ListModels;
                }

public class InsertDetails
    {        
        private string brandOfCar;
                
        public InsertDetails()
        { }

        public InsertDetails(string brandOfCar)
        {
            this.brandOfCar = brandOfCar;
        }


        public string BrandOfCar
        {
            get { return brandOfCar; }
            set { brandOfCar = value; }
        }  

 
        <asp:DropDownList ID="DDL_Brands" runat="server" 
            DataSourceID="ObjectDataSource1" Height="25px" Width="182px" 
            DataTextField="brandOfCar" DataValueField="brandOfCar" AutoPostBack="True">
            <asp:ListItem></asp:ListItem>
        </asp:DropDownList>
        &nbsp;&nbsp;&nbsp;&nbsp;    
        
        <asp:ObjectDataSource ID="ObjectDataSource1" runat="server" 
            SelectMethod="GetModels_DDL" TypeName="AutoCatalog.InsertDB">
            <SelectParameters>
                <asp:Parameter DefaultValue="Ford" Name="brandOfCar" />
            </SelectParameters>
        </asp:ObjectDataSource>

CREATE PROCEDURE GetModels_DDL
@brandOfCar nvarchar(30)
AS 
SELECT nameModel FROM Models where Models.brandId in(Select idBrand from Brands where Brands.brandOfCar = @BrandOfCar);