Error During SqlDataSource.Update() Instruction

Hi, I have such a problem:
I try to update (or insert) a row in my table and i fail althought i,ve read many posts here. I.ve created a button and "on_click" event to this button and want that event to update or insert a record in my table. I wrote:
 
protected void selectButton_Click(object sender, EventArgs e)
    {
        String taskID = projectsGridView.SelectedRow.Cells[0].Text;
        usersSqlDataSource.UpdateCommand = "update [Users] set [TaskID]=@task where [UserID]=1";
        usersSqlDataSource.UpdateParameters.Add("task", taskID);
        usersSqlDataSource.Update();
    }
 The application creates error in the last line of code (usersSqlDataSource.Update();) and i receive such an error:


You have specified that your update command compares all values on SqlDataSource 'usersSqlDataSource', but the dictionary passed in for oldValues is empty. Pass in a valid dictionary for update or change your mode to OverwriteChanges. For me it looks like there is a problem while setting parameters. Shall i change some properties of the sqlDataSource or GridView? Please help..