"It's really frustrating to be fair," Johnstone said in his post-match interview. "To concede with literally 20-30 seconds to go is a bit of a killer.
"But we knew today wasn't going to be easy. I think the majority of the game we played well.
"We could have had a number of goals but they weren't to be so it's obviously frustrating."
Luton equalised in the 96th minute, after Palace had dominated the majority of the game and looked the more likely side to add a second.
"We know how they are dangerous in the box," Johnstone said. "They had the two big lads up front towards the end, and it's a cross we should probably stop. It's just one of those things.
"We tried our best again but we knew it was going to be tough.
"If you don't score a couple of goals you always have that small percent [chance] of them coming and scoring, no matter who the team is.
"They gave it their all to the end and so did me. It was just one of those days. We could have had more goals, we didn't but at the end of the day we got a point and we move forward now."
