when i am retriving data from database i am getting following error "Unable to cast object of type 'System.String' to type 'System.Byte[]'."

SqlConnection connection = new SqlConnection("data source=.; initial catalog=jai; uid=sa; pwd=12345");
        string sql = "SELECT ProductName, ProductImage FROM image WHERE ImageID = @ImageID";
        SqlCommand cmd = new SqlCommand(sql, connection);
        cmd.CommandType = CommandType.Text;
        cmd.Parameters.AddWithValue("@ImageID", theID);
        connection.Open();
        object theImg = cmd.ExecuteScalar();
        try
        {
            return new MemoryStream((byte[])theImg);
        }
        catch
        {
            return null;
        }
        finally
        {
            connection.Close();
        }
    }