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After Nicolas Raskin's strike put the visitors ahead, Dan Neil's mistake gifted Dundee an equaliser.
Ferguson said on BBC Sportsound: "It's a mistake at the back for Rangers. It's a gift for Dundee.
"It's a blunder of a pass and Dundee take full advantage. It's Neil who loses his bearings a bit. He's just caught in two minds.
"I'm a bit shocked because Rangers have been so dominant in the second half. It's an awful mistake."
Rangers manager Derek McInnes compared the conceded goal to a balloon bursting.
He said: "The performance for 20-25 minutes culminated in Nico's goal. It should be a winning goal, because it was such a high-quality goal.
"The disappointment is that we had the game there, we were dominant. We had the game by the scruff of the neck. Dundee were struggling to get out of their own half.
"We were in such a good place. It was almost like a balloon popping when we made a mistake, which can happen.
"We never really got back to that same level. Although we did have enough balls in their box to score. We should be scoring.
"With some of the stats and some of the touches we had, we should be scoring more than two goals a night. So a combination of that and us also coming away from home, shouldn't need to score three goals to win a game of football."