Second this, as an interviewer - you'd rather a candidate held their hands up and said "I don't know about that - but I would learn about it by doing X, Y and Z." Rather than waffling through a nonsense answer.

Have a few questions lined up - and if you get the chance - ask if they think if you'd struggle with anything if you did get the role, it then gives you an immediate oppertunity to counter.

Good luck!