Kasey made a dash from his line just before half-time and was beaten to the ball by Thierry Henry, who was caught as he attempted to skip past the USA international. Moments before, after a long period of inactivity, Kasey pulled off his only save of the match to deny the Frenchman with a fine stop from a stooping header.
"It was just an error of judgement," said Glenn in his press conference after the game. "Henry was going nowhere, out to the corner flag.
"In saying that, probably the only chance they created in the game was just before that and Kasey's come up with a fantastic save.
"He's been great for us this year.
"I spoke to him after the game and he is obviously disappointed. It was a definite penalty though, no doubt about that.
"It was a blow for us, but we should have come in three or four goals up. I wouldn't say it changed the game because we still came out and went at it.
"For some teams it would have been such a blow that the performance would have declined. The passion was still there along with everything a North London derby needs.
"The only thing that was missing was a clinical finish."
Glenn went on to pay tribute to the spectacular free-kick strike from Christian Ziege, who registered his second goal in two games.
"It was a wonderful free-kick and it is the second week he has scored from one. It is always a handy thing to have up your sleeve.
"We caused them a lot of problems from set plays and it was there for all to see."
Glenn reflected that Kasey Keller simply made an 'error of judgement' in the challenge that led to Arsenal's penalty leveller, but pointed out that the keeper has been pretty much faultless all season.