City went into the game as strong favourites after beating Coventry at the weekend and thanks to their divisions advantage over Crewe, but home advantage and passion for the railwaymen paid dividence as the League One side look forward to a third round tie.
Johnson told supporters that his side were 'disappointed' but urged them to forget about defeat and focus on Saturday's game against QPR. He told the official website: "We're disapointed, the lads are disapointed but we need to work hard and make sure we don't have a 'dopey, posey' game again.
"We have to use this defeat and bounce back in the league but that doesn't make us feel any better after going out of the cup."