Error - Updating Tables Row Using SqlDataSource

I have such a problem:
i try to update a row in my table using:
    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();
    }
And i receive error on  usersSqlDataSource.Update():
You have specified that your update command compares all values on SqlDataSource 'usersSqlDataSource', but the dictionary passed in for oldValues is empty
What have i done wrong? Parameter are not set?