But Lambert is confident Butland will recover from the mistake.
"I think he's the best in Britain, no doubt, a huge goalkeeper and he will get better and better," Lambert enthused. "It was a brilliant save to his right (after the error) and he will bounce back.
"I think anybody who knows him knows he's very strong mentally and I've got a lot of time for him as a person too.
"I said to him `ok you made a mistake, no problem, but it's what you do from now on in.' But he's a top goalkeeper, a top professional.
"He's a good kid as well, he's not a big-time guy who walks around the place as if he owns it."